Repairing a 1960s-era IBM keypunch: controlled by mechanical tabs and bars

In this article I describe repairing an IBM 029 keypunch that wouldn't punch numbers. Keypunches were a vital component of punch card computing, recording data as holes in an 80-column card. Although keypunches have a long history, dating back to the use of punch cards in the 1890s, the IBM 029 keypunch is slightly more modern, introduced in 1964. The repair turned out to be simple, but in the process I learned about the complex mechanical process keypunches used to encode characters.

Keyboard of an IBM 029 keypunch. "Numeric" key is in the lower left. Strangely, this keyboard labels most special characters with the holes that are punched (e.g. 12-8-6) rather than the character. Compare with the photo of a different 029 keyboard later.

A couple weeks ago, I was using the 029 keypunch in the Computer History Museum's 1401 demo room and I found that numbers weren't punching correctly. The keyboard has a "Numeric" key that you press to punch a number or special character. (Unlike modern keyboards with a row of numbers at the top, numbers on the 029 keyboard share keys with letters.) When I tried to type a number, I got the corresponding letter; the keypunch was ignoring the "Numeric" key. The same happened with special characters that required "Numeric".

Frank King, an expert on repairing vintage IBM computers, showed me how to fix the keyboard. The first step was to remove the keyboard from the desk. This was surprisingly easy—you just rotate the keyboard clockwise and lift it up.

The keyboard of an IBM 029 keypunch can be removed from the desk simply by rotating and lifting.

On the underside of the keyboard are several microswitches for some special function keys. The microswitches are on the left half, connected by blue wires. Also note the metal rectangles along the right; these are the "latch contacts", one for each key and will be discussed later.

The underside of the IBM 029 keypunch's keyboard.

Frank noticed that the keystem for the "Numeric" key wasn't pressing the microswitch, but was out of alignment and missing the microswitch's lever entirely. Thus, pressing the "Numeric" key had no effect and the wrong character was getting punched. He simply rotated the microswitch slightly so it was back into alignment with the keystem, fixing the problem. We placed the keyboard back into the desk and the keypunch was back in business. Many vintage computer repairs are difficult, but this one was quick and easy.

How the keypunch encodes characters

This repair was a good opportunity to look inside the keyboard and study the interesting techniques it uses to encode characters. On a punch card, each character is indicated by the holes punched in one of the card's 80 columns, as shown below. The digits 0 through 9 simply result in a punch in row 0 through 9. Letters are indicated by a punch in digit rows 1 through 9 combined with a punch in one of the top three rows (the "zone" rows1). Special characters usually use three punches. Since each character can have punches in any of 12 rows, you can think of cards as using a (mostly-sparse) 12-bit encoding.

An 80-column punch card stores one character in each column. The pattern of holes in the column indicates the character. From the 029 Reference Manual.

Since each key on the keyboard has one character in alpha mode and another character in numeric mode, the keypunch must somehow determine the hole pattern for each character. With modern technology, you could simply use a tiny ROM to hold a 12-bit row value for the "alpha" mode and a second 12-bit value for the "numeric" mode. (Or use a keyboard encoding chip, digital logic, a microcontroller, etc.) But back in the keypunch era, ROMs weren't available. Instead, the keypunch uses a complex but clever mechanical technique to assign a hole pattern to each character.

The previous model: the 026 keypunch

IBM 026 keypunch. Photo by Paul Sullivan (CC BY-ND 2.0).

Before explaining how the 029 keypunch encodes characters, I'll discuss the earlier and simpler IBM 026 keypunch, which was introduced in July 1949. The 026 used the technology of the 1940s: vacuum tubes and electromechanical relays. Encoding the hole pattern with tubes or relays would be bulky and expensive. Instead, the keypunch used a mechanical encoder with metal tabs to indicate where to punch holes.

The keyboard mechanism in the 026/029 keypunch. Pressing a key pulls the latch pull-bar. This causes the permutation bar to drop slightly. If a bail has a matching tab, the permutation bar will move the bail, closing the contact. The permutation bar also closes the key's latch contact. Based on the Maintenance Manual.

The diagram above shows the keyboard mechanism in the keypunch. The basic idea is there are 12 "bail contacts" (one for each row on the card); when you press a key, the appropriate contacts are closed to punch holes in the desired rows. To implement this, each key was connected to a separate vertical "permutation bar" by a "latch pull-bar". When a key is pressed, the associated permutation bar drops down. Twelve horizontal bars, called "bails",2 ran perpendicular to the permutation bars, one bail for each row on the card. At each point where a bail crossed a key's permutation bar, the bail could have a protruding tab that meshes with the permutation bar. Pressing a key would cause the permutation bar to push the tab and rotate the bail, closing the bail contact and punching a hole in that row. Thus, the 12 bails mechanically encoded the mapping from keys to holes by the presence or absence of metal tabs along their length.

Closeup of the bails and one of the permutation bars. Four of the bails have tabs and will be triggered by the permutation bar.

The photo above shows a permutation bar (right) engaging four tabs on the bails (left). In the photo below, you can see one of the bails removed from the keyboard mechanism. Note the tabs extending from the bail to engage with the permutation bars. Also note the contact on the left end of the bail.

The keyboard mechanism in the 029 keypunch. From the Maintenance Manual.

There is a problem with this 12-bail mechanism: it only handles a single character per key, so it doesn't handle numbers. (The keyboard diagram below shows how numbers share keys with letters.) The obvious solution is to add 12 more bails for the second character on a key, but this would double the cost of the mechanism. However, since numbers are indicated by a single punch in a column, a shortcut is possible: use a switch on each number key to punch that row. The 026 keypunch does this, using the latch contacts shown in the earlier diagram. In numeric mode, the latch contact for the "1" key would punch row 1, and so forth for the other numbers. To handle the special characters, three additional bails were added, bringing the total number of bails to 15.5 Thus, the 026 had 12 bails used in alpha mode, 3 bails used in numeric mode for special characters, and a latch contact for each key for numbers and special characters.

Keyboard of the IBM 026 keypunch. From the IBM 24/26 Reference Manual.

The permutation bar and bail mechanism also explains why the "Numeric" key (the one we fixed) has a separate microswitch under the keyboard. The regular mechanism with permutation bars, bails and latch contacts only allows one key to be pressed at a time.3 Since "Numeric" is held down while another key is pressed, it needed a separate mechanism.4

Back to the 029 keypunch

When the 029 keypunch was introduced in 1964, it replaced the 026's vacuum tubes with transistorized circuitry and updated the keypunch's appearance from 1940's streamlining to a modernist design. However, the 029 kept most of the earlier keypunch's internal mechanical components, along with many relays.6

Keyboard from the IBM 029 keypunch. Photo by Carl Claunch.

A major functional improvement over the 026 was the addition of many more special characters in the 029; almost every key on the keyboard had a special character. The three numeric mode bails used in the 026 couldn't support all these special characters. The obvious solution would be to add more bails; with 12 bails for alpha and 12 bails for numeric, any characters could be encoded. But, IBM came up with a solution for the 029 that supported the new special characters while still using the 026's 15-bail mechanical encoder. The trick was to assign the bails to rows in a way that handled most of the holes, leaving the latch contact to handle one additional "extra" hole per key.7

The diagram below shows part of the encoding mechanism, based on the keypunch schematics.8 Horizontal lines correspond to the 15 bails; the labels on the left show the row handled by each bail. Each vertical line represents the permutation bar for a key; the key labels are at the bottom. The black dots correspond to tabs on the bail, causing the bail to tripped when activated by the corresponding key. (The circles symbolize the interlock disks that keep two keys from being used simultaneously.3) Note that the 029's bails don't handle all the rows for alpha mode (unlike the 026), leaving some alpha holes to be punched by the latch contacts.

Diagram showing how keys select holes to be punched on the 029 keypunch. The complete diagram is in the schematic.

The yellow highlights on the diagram show what happens for the "W _" key. Pressing this key (bottom) will activate the "Numeric 5", "Numeric 8" and "Common 0" bails (left). In addition, the latch contact will activate "Alpha 6" (top). Putting this together, in alpha mode, rows 0 and 6 will be punched and in numeric mode, rows 0, 8 and 5 will be punched. These are the codes for "W" and "_" respectively, so the right holes get punched. The encoder works similarly for other keys, punching the appropriate holes in alpha and numeric modes. Thus, the 029 managed to extend the 026's character set with many new special characters, without requiring a redesign of the mechanical encoder.

An IBM 029 keypunch in the middle of punching cards.

Conclusion

For once, repairing computer equipment from the 1960s was quick and easy. Fixing the "Numeric" key didn't even require any tools. The repair did reveal the interesting mechanism used to determine which holes to punch. In the era before inexpensive ROMs, keyboard decoding was done mechanically, with tabs on metal bars indicating which holes to punch. This mechanism was inherited from the earlier 026 keypunch (1949), improved on the 029 keypunch (1964) to handle more symbols, and was finally replaced by electronic encoding when the 129 keypunch was introduced in 1971.6

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Notes and references

1. The card's zone rows are called 12 (on top) and 11 (below 12). Confusingly, sometimes the 0 row acts as a zone (for a letter) and sometime it acts as a digit (for a number). Also note the special characters that use 8 combined with a digit from 2 to 7; essentially this corresponds to a binary value from 10 to 15. The combination of a zone punch and digit punches encoded a 6-bit character. ↩

2. "Bail" may seem like an obscure word in this context, but it's essentially a metal bar. Looking at old patents, in the early 1900s "bail" was most often used to denote a wire handle on a bucket or pot. It then got generalized to a metal bar in various mechanism, especially one that could be lifted up. If you're from the typewriter era, you might remember the "paper bail", the bar that holds the paper down. (Dictionary link.) ↩

3. An interesting mechanism ensures that only one key can be pressed at a time. The keyboard mechanism contains a row of "interlock disks". When a key is pressed the latch bar slides between two of these disks. These disks slide all the other disks over, blocking the path of any other key's latch bar until the first key is released. ↩

4. The other special keys that use a microswitch are "Error reset", "Multi punch", "Dup", "Feed", "Prog 1", "Prog 2" and "Alpha". ↩

5. The 026 keypunch used a clever trick for some of the special characters. Note the four special character keys in the upper left. These characters were carefully assigned so each pair has the same punch except the upper one punches a 3 and the lower punches a 4. Thus, a single encoding worked for the key with the addition of a relay to switch between 3 and 4. ↩

6. In 1964 IBM introduced the IBM 360 line of computers. They were built from hybrid SLT (Solid Logic Technology) modules, an alternative to integrated circuits. Although the IBM 029 keypunch was introduced along with the SLT-based IBM 360, the keypunch used older transistorized SMS boards. Even though the 029 got rid of the 026's vacuum tubes, it was still a generation behind in technology. It wasn't until the 129 keypunch was announced in 1971 (along with the 370 computer line) that keypunches moved to SLT technology.

   The 129's keyboard kept much of the mechanical structure of the older keypunches (the permutation bars and latch contacts), but got rid of the bails used to encode the keypresses. Instead, encoding in the 129 was done through digital logic SLT modules (mostly AND-OR gates) controlled by the latch contact switches. The 026 and 029 keypunches originally had model numbers 26 and 29, but with the introduction of the 129, their names were retconned to the 026 and 029. ↩

7. It's not easy to design a keyboard layout that works with the 029 mechanism since the latch contact can support at most one "extra" hole per key, a hole not handled by the bails. The special characters needed to be assigned to keys in a way that would work; this probably explains the semi-random locations of the special characters on the keyboard. For instance, "(" is on "N" while ")" is on "E". Since both ")" and "E" require a hole in row 5, it made sense to put them on the same key, so the latch contact can handle the 5 punch for both. ↩

8. If you want to learn more about keypunch internals, the manuals are on bitsavers.org. In particular, see the Reference Manual, Maintenance Manual and Schematics. ↩

Creating a Christmas card on a vintage IBM 1401 mainframe

I recently came across a challenge to print a holiday greeting card on a vintage computer, so I decided to make a card on a 1960s IBM 1401 mainframe. The IBM 1401 computer was a low-end business mainframe announced in 1959, and went on to become the most popular computer of the mid-1960s, with more than 10,000 systems in use. The 1401's rental price started at $2500 a month (about $20,000 in current dollars), a low price that made it possible for even a medium-sized business to have a computer for payroll, accounting, inventory, and many other tasks. Although the 1401 was an early all-transistorized computer, these weren't silicon transistors—the 1401 used germanium transistors, the technology before silicon. It used magnetic core memory for storage, holding 16,000 characters.

A greeting card with a tree and "Ho Ho Ho" inside, created on the vintage 1401 mainframe. The cards are on top of the 1403 line printer, and the 1401 mainframe is in the background.

You can make a greeting card by printing a page and then folding it into quarters to make a card with text on the front and inside. The problem with a line-printer page is that when you fold it into a card shape, the printed text ends up sideways, so you can't simply print readable text. So I decided to make an image and words with sideways ASCII graphics. (Actually the 1401 predates ASCII and uses a 6-bit BCD-based character set called BCDIC, so it's really BCDIC graphics. (EBCDIC came later, extending BCDIC to 8 bits and adding lower case.)) Originally I wanted to write out "Merry Christmas", but there aren't enough characters on a page to make the word "Chrstmas" readable, so I settled on a cheery "Ho Ho Ho". I figured out how to sideways draw a tree and the words, making this file.

Closeup of a greeting card printed on the IBM 1401, with a Christmas tree on the front.

Next, I needed a program to print out this file. I have some experience writing assembly code for the IBM 1401 from my previous projects to perform Bitcoin mining on the 1401 and generate Mandelbrot fractals. So I wrote a short program to read in lines from punched cards and print these lines on the high-speed 1403 line printer. The simple solution would be to read a line from a card, print the line, and repeat until done. Instead, I read the entire page image into memory first, and then print the entire page. The reason is that this allows multiple greeting cards to be printed without reloading and rereading the entire card deck. The second complication is that the printer is 132 columns wide, while the punch cards are 80 columns. Instead of using two punch cards per print line, I encoded cards so a "-" in the first column indicates that the card image should be shifted to the right hand side of the page. (I could compress the data, of course, but I didn't want to go to that much effort.)

The 1401 has a strange architecture, with decimal arithmetic and arbitrary-length words, so I won't explain the above code in detail. I'll just point out that the r instruction reads a card, mcw moves characters, w writes a line to the printer, and bce branches if a character equals the specified value. (See the reference manual for details.)

The next step was to punch the code and data onto cards. Fortunately, I didn't need to type in all the cards by hand. Someone (I think Stan Paddock) attached a USB-controlled relay box to a keypunch, allowing a PC to punch cards.

A PC-controlled IBM 029 keypunch punched my card deck.

A few minutes later I had my deck of 77 punch cards. The program itself just took 9 cards; the remainder of the cards held the lines to print.

The deck of punched cards I ran on the IBM 1401. The first few cards are the program, and the remaining cards hold the lines to print.

Once the cards were ready, we loaded the deck into the card reader and hit "Load", causing the cards to start flying through the card reader at a dozen cards per second. Unfortunately, the reader hit an error and stopped. Apparently the alignment of the holes punched by the keypunch didn't quite match the alignment of the card reader, causing a read error.

The IBM 1401's card reader was experiencing errors, so we removed the brushes and realigned them.

The card reader contains sets of 80 metal brushes (one for each column of the card) that detect the presence of a hole. Computer restoration expert Frank King disassembled the card reader, removed the brush assembly from the card reader and adjusted it.

A closeup of the brush module with 80 brushes that read a card.

After a few tries, we got the card reader to read the program successfully and it started executing. The line printer started rapidly and noisily printing the lines of the card. We had to adjust the line printer's top-of-form a couple times so the card fit on the page, but eventually we got a successful print.

Printing a greeting card on the IBM 1403 line printer.

I ejected the page from the printer, tore it off, and folded the card in quarters, yielding the final greeting card. It was a fun project, but Hallmark still wins on convenience.

Greeting card created by the IBM 1401 mainframe (background).

If you want to know more about the IBM 1401, I've written about its internals here. The Computer History Museum in Mountain View runs demonstrations of the IBM 1401 on Wednesdays and Saturdays. It's amazing that the restoration team was able to get this piece of history working, so if you're in the area you should definitely check it out; the demo schedule is here.

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Identifying the "Early IBM Computer" in a Twitter photo: a 405 Accounting Machine

The photo below of a "woman wiring an early IBM computer" has appeared on Twitter a bunch of times, and it stoked my curiosity: what was the machine in the photo? Was it really an early IBM computer? I was a bit skeptical since the machine in the photo is much smaller than IBM's first room-filling computers, and there aren't any vacuum tubes visible. I investigated this machine and it turned out to be not a computer, but an IBM 405 "Alphabetic Accounting Machine" from 1934, back in the almost forgotten pre-computer age of tabulating machines.1

A common photo on Twitter shows a woman wiring an early IBM computer.

The image is from photographer Berenice Abbott who took many scientific photographs from 1939 to 1958. She photographed everything from basic magnetic field and optics experiments to research on early television tubes, and many of these photos were used in physics textbooks. Her photos (including IBM photos) were published in the book Documenting Science. I had hoped that the book would identify the computer in the photo, but it merely said "IBM Laboratory: Wiring an early IBM computer". Surprisingly for an art book, it didn't even give a date for the photo.

The diagram below shows the back view of an IBM 403 accounting machine, which IBM introduced in 1948.2 (An accounting machine (also called a tabulator) summed up and printed records from punched cards for applications such as payroll or inventory.) Note the similarities with the Abbott photo: the thick laced wire bundles, the vertical wire bundles in the middle for the counters, and the hinged doors that swing open.

Back view of an IBM 403 accounting machine. From the IBM 402/403 Field Engineering Manual.

A second Berenice Abbott photo shows the machine from a slightly different angle.3 The "Card Feed Circuit Breaker Unit" in the upper right looks like a perfect match between the IBM 403 and the Abbott photo. The dangling cables from the counters in the middle look right, as well as the thick cable between the counters and the circuit breaker unit. The 403 diagram above shows a large printing carriage on top, while the Abbott photo just shows a base, presumably because the carriage hadn't been installed yet.

A second photo of "Woman wiring an early IBM computer" by Berenice Abbott.

Although the machine in the Abbott photo looks very similar to the IBM 403, there are a few differences if you look carefully. One clear difference is the IBM 403 had caster wheels attached directly to the frame, while the machine in the photos has stubby curved legs. In addition, the doors of the IBM 403 were hinged at a different place. In the Abbott photos, the doors are attached just to the left of the counters and to the right of the card feed circuit breaker unit. But the IBM 403 has some bulky components to the left of the counters such as the "Bijur pump" (an oil pump), and components on the right such as the drive motor. Overall, the machine in the Abbott photos has a narrower cabinet than the IBM 403. Additionally, the thick cable snaking down between the IBM 403's circuit breaker units appears to go straight down in the photos. Thus, although the machine in the photos is very similar to the IBM 403, it's not an exact match.

After more research into IBM's various accounting machines, I conclude that machine in the photos is the IBM 405, an IBM accounting machine introduced in 1934 (earlier than that 1948 IBM 403 despite the larger model number).4 The IBM 405 (below) had curved legs that match the Abbott photos. In addition, the 405 has a narrower main cabinet than the 403, with bulky additional components attached to the left and right, outside the legs. This matches the narrower cabinet in the Abbott photos. (The 403 was an improved and modernized 405, explaining the overall similarity between the two machines.)

An IBM 405 accounting machine. Photo courtesy of Columbia University Computing History.

Punched cards were a key part of data processing from 1890 until the 1970s, used for accounting, inventory, payroll and many other tasks. Typically, each 80-column punched card held one record, with data stored in fixed fields on the card. The diagram below shows a typical card with columns divided into fields such as date, vendor number, order number and amount. An accounting machine would process these cards: totaling the amounts, and generating a report with subtotals by account and department, as shown below.

Example of a punched card holding a 'unit record', and a report generated from these cards. The accounting machine can group records based on a field to produce subtotals, intermediate totals, and totals. From Manual of Operation.

Punched-card data processing was invented by Herman Hollerith for the 1890 US census, which used a simple tabulating machine to count census data, stored on punched cards. Tabulating machines steadily became more complex, becoming feature-laden "accounting machines" that could generate business reports. Businesses made heavy use of these electromechanical accounting machines and by 1944, IBM had 10,000 tabulating and accounting machines in the field.

Accounting machines were "programmed" with a removable plugboard. By switching the plugboard, an accounting machine could be rapidly reconfigured for different tasks. Each wire corresponded to one character or digit. Wires plugged into the plugboard connected columns on the input card to adders. Each column on the printer had an associated wire controlling what got printed. Other wires had control functions. (I explained the tax preparation plugboard below in detail in this article.)

Plugboard to generate a tax report on an IBM 403 accounting machine. Courtesy of Carl Claunch.

The IBM 405 was IBM's first "Alphabetic Accounting Machine," able to print text as well as numbers. It had more complexity than you might expect from the 1930s, able to generate three levels of subtotals, intermediate totals, and grand totals. It could process up to 150 cards per minute; that's remarkably fast for an electromechanical system, reading and summing more than 2 cards per second. The 405 was IBM's flagship product for many years, with IBM manufacturing 1500 of them per year. Like most IBM machines, the 405 was usually rented rather than purchased; it cost over $1000 a month (equivalent to about $15,000 per month in 2017 dollars). Renting out these machines (and selling the punch cards) was highly profitable for IBM, with the IBM 405 accounting machine called "the most lucrative of all IBM's mechanical glories".5

Amazingly, although accounting machines were designed for business purposes, they were also used for scientific computation in the 1930s and 1940s, before digital computers existed. They solved everything from differential equations and astronomy computations to nuclear bomb simulations for the Manhattan Project.6

How does an accounting machine work and what are all those parts?

Accounting machines (also called tabulators) were built from electromechanical components, rather than transistors or even vacuum tubes. The main components in accounting machines were electromechanical counters and relays. Everything was synchronized to rotating driveshafts that ran the counters, card reader and printer. In a way, accounting machines were more like cars than computers, with a motor, clutches, driveshafts, an oil pump, gears and cams.

Counters

The heart of the accounting machine was the mechanical counter, a digit wheel somewhat like an odometer. Each wheel stored one digit, with the value indicated by the rotational position of the wheel. To add a number, say 3, to the counter, a clutch was briefly activated, causing the drifeshaft to rotate the counter three more positions. Since these counters were adding 2 1/2 numbers per second, they were spinning rapidly with the clutches engaging and disengaging with precision timing. By combining multiple counters, numbers of up to 8 digits could be handled. The counter kept a running total of the numbers fed into it. Since it accumulated these numbers, the counter was known as an accumulator, a term still used in computing.

A counter unit from an IBM accounting machine. The two wheels held two digits. The electromagnets (white) engaged and disengaged the clutch so the wheel would advance the desired number of positions.

The photo above shows a board with two counters: the two wheels on the left stored two digits. The counters are more complex than you might expect, with electromechanical circuits to handle carries (including fast carry lookahead). The clutch is underneath the wheel and is engaged by the metal levers in the photo, controlled by electromagnets. A gear underneath the clutch connects the counter to the driveshaft. The electrical connections on the right control the clutch and allow the values from the counters to be read out. Since the IBM 405 had 16 accumulators, with up to 8 digits, many counters were required, resulting in the mass of counter wires in the photos.

Relays

Another key component of the accounting machine was the relay, an electromagnetic switch. The control logic of the accounting machine was implemented with hundreds of relays, which would turn on and off to direct the various components of the accounting machine. Example relay functions are switching on when punched cards are in the input hopper, selecting addition or subtraction for a counter, generating the final total when all cards are done, or printing a credit symbol for a negative balance.

The back of the IBM 403 accounting machine shows numerous relays, used to control the machine.

The relays were mounted on swing-out panels. The photo above shows an IBM 403 with the panels closed. In the Abbott photos the relay panels are opened and you can see the extensive wiring that connected the relays to the rest of the system.

Circuit breakers

The final component I'll explain is the "circuit breaker," which has nothing to do with the circuit breakers in your house. Instead, these are cam-controlled switches that turned on and off (breaking circuits) as the drive shafts rotated. Dozens of circuit breakers provided the timing signals to the accounting machine, ensuring all operations in the machine were synchronized to the drive shaft. (Every 18° of drive shaft rotation corresponded to reading one row on a punched card, moving one character position on a printer typebar, or advancing a counter wheel by one position.)

Conclusion

The woman in Abbott's photos illustrates the large, but mostly ignored role that women played in electrical manufacturing. Women formed the majority of workers in the 1920s radio manufacturing industry, and their presence in electrical manufacturing increased even more when World War II led many women to take industrial jobs. The famous ENIAC computer (1946) also illustrates this: most of the "wiremen" assembling the ENIAC computer were in fact women, probably recruited from the telephone company or radio assembly.8

The photos also provide a glimpse into the era before digital computers, when businesses used electromechanical accounting machines and tabulators for their data processing. Although you'd expect a machine from 1934 to be primitive, the IBM 405 accounting machine in the photos was an advanced piece of technology for its time, containing 55,000 parts and 75 miles of wire.5 These punched card machines were also capable of performing complex scientific tasks, even contributing to the Manhattan Project. In the 1960s, businesses gradually switched from accounting machines to stored-program business computers such as the IBM 1401. Even so, IBM continued marketing accounting machines until 1976.

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Notes and references

1. By the "era before computers", I mean before digital electronic computers came along in the 1940s. There's a long history of computing machines, analog computers, and human computers (as shown in Hidden Figures) before digital computers came along. Two interesting books that cover this history are The First Computers—History and Architecture or The Computer: From Pascal to von Neumann. ↩

2. IBM's 402 and 403 accounting machines were the same except the 403 could print three-line addresses. This feature was called MLP (multi-line printing) and was useful for printing addresses on invoices, for instance. So when I refer to the IBM 403 accounting machine, I'm also including the IBM 402. ↩

3. I was surprised to realize that there are two different, but nearly identical photos of the woman wiring the IBM machine floating around. In first photo, the woman's right leg is straight, there's a screwdriver in front of the bench and she's wiring the left side of the machine. In the second, the woman's left leg is straight and she's wiring the middle of the machine. ↩

4. I couldn't find any manuals for the IBM 405 or photos of the back of a 405, so I can't totally nail down the identification. There's a possibility that the Abbott photos show an IBM 401 accounting machine (below), which was similar to the 405 but introduced a year earlier. The IBM 401 and IBM 405 both had the same basic shape and arrangement of components. The main difference is 405 had a full cabinet in front while the 401 was empty in the front with a bar bracing the front feet. The Abbott photos seem to show a full cabinet like the 405, rather than the open 401. Also, since the 405 was a much more popular machine than the 401—the 405 was the "flagship of IBM's product line until after World War II"—the photos are most likely the 405.

   ↩

   IBM 401 accounting machine (1933). Photo courtesy of Computer History Museum.

5. Several sources provide information on the IBM 405. One source is Computing before Computers (online) p144. IBM has a FAQ with a short overview. Columbia's Computer History page on the 405 has a longer discussion. Also see IBM's Early Computers, pages 18-22 for information on the IBM 405. Computer: Bit Slices from a Life has some hands-on stories of the IBM 405 (online). Computer: A history of the information machine page 51 has some information on the IBM 405. ↩

6. See Punched Card Methods in Scientific Computation, 1940 for details on how accounting machines were used for scientific computation. The book is by W. J. Eckert, confusingly unrelated to the ENIAC's J. Presper Eckert. The use of IBM accounting machine by the Manhattan Project is described by Feynman in Los Alamos from Below, p25; and in this page. The Manhattan Project used a 402 accounting machine, several IBM 601 multiplying punches for multiplication, and other card equipment (reference, with more details in Early Computing at Los Alamos). ↩

7. Bitsavers has manuals for the later 402/403 accounting machines. The operation of accounting machines is discussed in detail in IBM 402, 403 and 419 Accounting Machines: Manual of Operation. For a thorough discussion of how the machine works internally, see IBM 402, 403, 419 Field Engineering Manual of Instruction. For an overview of how plugboard wiring for IBM's works, see IBM Functional Wiring Principles. ↩

8. Women and Electrical and Electronics Manufacturing provides a detailed discussion of the history of women in technological manufacturing in the 20th century. The book ENIAC in Action describes how most of the wiring of the ENIAC was done by women. It also has a detailed discussion of the role of women as programmers for early computers such as the ENIAC. ↩

One-hour Mandelbrot: Creating a fractal on the vintage Xerox Alto

I wrote a short program to generate the Mandelbrot set on the Xerox Alto, a groundbreaking minicomputer from the 1970s. The program, in the obsolete BCPL language, ran very slowly—taking almost exactly an hour—but the result below shows off the Alto's monochrome bitmapped display. (Bitmapped displays were a rarity at the time because memory was so expensive.)

The Xerox Alto took an hour to generate the Mandelbrot set.

The Alto was a revolutionary computer designed at Xerox PARC in 1973 to investigate personal computing. It introduced the GUI, Ethernet and laser printers to the world, among other things. In the photo above, the Alto computer itself is in the lower cabinet. The Diablo disk drive (with the 1970s orange stripe) uses a removable 14 inch disk pack that stores 2.5 megabytes of data. (A bunch of disk packs are visible behind the Alto.) The Alto's display is bitmapped with 606x808 pixels in an unusual portrait orientation, and the optical mouse is next to the display.

Last year Y Combinator received an Alto from computer visionary Alan Kay and I'm helping restore the system, along with Marc Verdiell, Luca Severini and Carl Claunch. My full set of Alto posts is here and Marc's videos are here. I haven't posted an update for a while, but now I can write new programs and download them to the Alto using the Living Computer Museum's Alto file system implementation and gateway to the Alto's 3Mb Ethernet. I decided to start with the Mandelbrot set to take advantage of the Alto's high resolution display.

Marc's latest video shows the Mandelbrot programming running on the Alto.

The Mandelbrot program

The Mandelbrot set algorithm is fairly simple. Each point on the plane represents a complex number c. You repeatedly iterate the complex function f(z) = z^2 + c. If the value diverges to infinity, the point is outside the set. Otherwise, the point is inside the set and the pixel is set to black. Setting the pixel is tricky because the Alto doesn't have a graphics API; you need to determine which bit in memory to set.4

Since the Xerox Alto doesn't support floating point1, I needed a way to represent the numbers with its 16-bit word. I use fixed point arithmetic: 4 bits to the left of the decimal point and 12 bits to the right.2 For instance, the number 1.25 is represented in 16 bits as 1.25*2^12 = 0x1400. These fixed point numbers can be added with standard integer addition. After multiplying two fixed point numbers with integer multiplication, the 32-bit result must be divided by 2^12 (i.e. shifted right by 12) to restore the decimal point location.3

The code (above) is written in BCPL, the main language used on the Alto. BCPL is a precursor to C and many features of C are clearly visible in BCPL: everything from lvalues and rvalues to the ternary operator. You can think of BCPL as C without types; the only BCPL types are 16-bit words along with C-like structs, unions and bitfields. BCPL may look unfamiliar at first, but the code above should be clear if you consider the following syntax differences with C:

• Blocks are indicated with [ and ] instead of { and }.
• Indexing is with a!1 instead of a[1].
• And, Or, and Shift bit operations are &, %, and lshift/rshift.
• Variable definitions use let.
• Arrays are defined with vec.

More information on BCPL is in the BCPL Reference Manual and my earlier article on using BCPL with the Alto.

The Xerox Alto, a few minutes into generation of the Mandelbrot set.

Why is the Alto so slow?

Running the Mandelbrot set illustrates the amazing improvement in computer speed since the Alto was created in 1973 and the huge changes in computer architecture. On a modern computer, a Javascript program can generate the Mandelbrot set in a fraction of a second, while the Alto took an hour. The first factor is the Alto's slow clock speed of 5.88 MHz, hundreds of times slower than a modern processor. In addition, the Alto doesn't execute machine instructions directly, but uses a relatively inefficient microcode emulator that takes many cycles to perform one machine instruction.

The ALU board from the Xerox Alto. The Alto doesn't use a microprocessor chip, but a CPU built from three boards of integrated circuits.

Unlike modern computers, the Alto doesn't use a microprocessor chip, but instead has a CPU built from three boards full of simple TTL chips. The photo above shows the arithmetic-logic unit (ALU) board, which uses four 4-bit 74181 ALU chips to perform addition, subtraction and logic operations. You can also see the CPU's registers on this board. The Alto doesn't include a hardware multiplier, but must perform multiplication by repeated shifts and adds. Thus, the Alto performs especially poorly on the Mandelbrot set, which is essentially repeated multiplications.

Conclusion

The Mandelbrot set was a quick program to try out the Alto's graphics. Next I'll try some more complex projects on the Alto. If you want to run my code, it's on Github; you can run it on the ContrAlto simulator if you don't have an Alto available.

If you're interested in retrocomputing fractals, I also generated a Mandelbrot on a 50 year old IBM 1401 mainframe The 1401 generated the Mandelbrot set in 12 minutes—not because it's a faster machine than the Alto, but because the resolution on the line printer was very very low.

Mandelbrot generated on the IBM 1401 mainframe.

Notes and references

1. There is a floating point library (source) for the Alto. I decided not use use it since the integer Mandelbrot was already very slow. But using floating point would make sense if you wanted to zoom in on the Mandelbrot. ↩

2. Fixed-point arithmetic is a common trick for fast Mandelbrot calculation. ↩

3. To multiply two 16-bit numbers efficiently, I use the double precision MulFull function (written in Nova assembler) in PressML.asm, part of the Computer History Museum's archived Alto software. ↩

4. The hardest part of generating the Alto Mandelbrot was figuring out how to configure the display memory and update it correctly. The details on how the display works are in chapter 4 of the Xerox Alto Hardware Manual. To summarize, the display contents are defined by a linked list of display control blocks (DCBs), which define a rectangular region of pixels on the display. A microcode task reads 16 words of pixels from memory at a time and writes them to the display board, which shifts the pixels out to the monitor. Thus, as each scanline is being written to the CRT, the CPU is busily reading the pixels for that line from memory and feeding them to the display, another reason why the Alto is slow.

   The Alto's Smalltalk environment has a simple graphics API, but we don't have Smalltalk running yet. ↩

1950's tax preparation: plugboard programming with an IBM 403 Accounting Machine

Long before computers existed, businesses used electromechanical accounting machines for data processing. These one-ton accounting machines were "programmed" through wiring on a plugboard control panel, allowing them to generate complex business reports from records stored on punched cards. Even though they lacked electronics and used spinning mechanical wheels to add up data, these machines could process more than two cards a second.

This plugboard for an IBM 403 implements tax deduction computation. Board courtesy of Carl Claunch.

In honor of April 151, I examine a plugboard that was used for tax preparation in the 1950s9 and explain the forgotten art of plugboard programming, showing how a tangle of wiring implemented a data processing algorithm. By mounting the plugboard on an accounting machine, a particular data processing task could be performed. Although the plugboard looks like spaghetti code made physical, tracing out the connections shows its function: it computed deductions by summing records across multiple fields, printed a report with subtotals and totals, and punched a smaller card deck with the subtotals.

Overview of punched card data processing

Punched cards were a key part of data processing from 1890 until the 1970s, used for accounting, inventory, payroll and many other tasks. Typically, each 80-column punched card held one record, with data stored in fixed fields on the card. The example below shows an example card with columns divided into fields such as date, vendor number, order number and amount. An accounting machine would process these cards: totaling the amounts, and generating a report with subtotals by account and department, as shown below.

Example of a punched card holding a 'unit record', and a report generated from these cards. The accounting machine can group records based on a field to produce subtotals, intermediate totals, and totals. From Manual of Operation.

Punched-card data processing was invented by Herman Hollerith for the 1890 US census, which used a simple tabulating machine that counted records indicated by holes in the cards.2 These machines steadily accumulated features, becoming complex "accounting machines" that could generate business reports.6 These machines became popular with businesses and by 1944, IBM had 10,000 tabulating and accounting machines in the field.3 In July 1948, IBM introduced the 402 Accounting Machine, which used the plugboard I'm examining. The 402 (and the similar 4035) were feature-rich machines that had 16 counters, multiple levels of subtotals, vertical spacing control to support forms, comparisons and conditional operations, and leading zero elimination.

IBM 403 accounting machine, with Type 82 card sorter at right.4 These machines are on display at the Computer History Museum.

The surprising thing about this history is that businesses were performing data processing with punched cards decades before the first computers, using machinery that was entirely electro-mechanical, not even using vacuum tubes. This equipment was built from components such as wire brushes to read holes in punch cards, relays to control the circuits, and mechanical counter wheels to add values. Even though these systems were technologically primitive, they revolutionized business data processing and paved the way for electronic business computers such as the popular IBM 1401.

Plugboard programming

The accounting machines were programmed by wiring up a plugboard for a specific task. Since each application used cards with fields in different positions, accounting machines needed a way to define each field. Different reports would be formatted with values in different locations on the page. Applications would need to total and subtotal different values. Before stored-program computing existed, a technique was needed to easily customize the system for a particular application. The result was wiring on control panel plugboards.

Closeup of the plugboard for an IBM 403. The accounting machine is "programmed" by plugging in wires to form connections.

The photo above shows a closeup of the plugboard. The plugboard has a grid of holes (which are called hubs), with their functions labeled. By inserting a wire into the board, two hubs are connected, causing the accounting machine to perform a particular operation. The collection of wires specifies the operations that are performed on each card.

The back of the plugboard for an IBM 403 accounting machine.

When a wire is inserted into the plugboard, the jack on the end of the wire sticks out the back of the plugboard, as shown above. When the plugboard is mounted in the accounting machine (below), these jacks make contact with a grid of connectors on the accounting machine, completing the desired circuits. (Note the "setup change" switches above the plugboard; these switches will be relevant later.)

A plugboard inserted into the side of an IBM 403 at the Computer History Museum. Note the control switches above the plugboard. These can be used to change what the plugboard does.

Since the plugboard is removable, companies could easily switch plugboards to perform different tasks. (Rewiring a plugboard for each function would be much too time-consuming.) As a consequence, companies might have shelves full of plugboards for all the operations they performed; with plugboards, the "software" takes up considerable physical space. The photo below shows one company's collection of plugboards to perform different tasks.

Shelves full of plugboards for the IBM 402, courtesy of IBM 1401 restoration team.

The tax program

I closely examined the wiring of the tax plugboard to determine what it does. The first step was to trace out each wire to draw a schematic wiring diagram (below) that shows all the connections on the plugboard. If you compare the diagram with the plugboard photo at the start of the article, you can see that it shows the same wiring, but in a much easier to follow format.

A wiring diagram for an IBM 403 plugboard to compute tax deductions. (Click for full size.)

I found that the program wired into the board reads cards and computes subtotals and totals from the cards. In more detail, each card has seven fields that are read. The first field is an identifier, and all cards with the same identifier are totaled together to give totals for each of five fields. My hypothesis is that this field is an employee id, and each card corresponds to one pay period.7 Summing the records for each employee id gives the employee's total deductions (or year-to-date deductions). The totaled five fields could be payroll deductions such as federal income tax, state tax, social security tax, Medicare tax and retirement contributions. After reading the cards for an employee, the accounting machine punches a new summary card with the employee's total deductions prints a line on the report. The per-employee totals are then summed together to give overall totals at the end.

Here's how the plugboard works, step by step. When an 80-column card is read, each digit is available in one of the reading hubs, labeled 1 through 80. By putting a wire in a hub, the digit is transmitted to another part of the machine. For instance, suppose there is a 6-digit number punched into columns 28 to 33 of the card and we want to total these numbers. This is done by connecting a wire from reading column 28 to the upper digit of the counter, a wire from column 29 to the second digit of the counter, and so forth, for 6 wires in total.

The wires transferring the field to counter 6C are the six red wires in the photo below. The 80 card columns are available in the two rows of hubs below the label "Third reading". The inputs to the counters are the four rows of hubs below the "Counter entry" labels. Other fields are wired to counters similarly.

The six red wires connect six columns read from the card (right) to the entry of counter 6C (left).

Trying to figure out the wiring from the photo is difficult, so plugboard wiring is typically indicated in a diagram. The diagram below shows the wiring between the columns read (right) and the counter 6C (left). The six wires are compressed into one line on the diagram, using IBM's style of representing plugboards. The horizontal bars connected by a line indicate six parallel wires.

A diagram representing the connection between the card read (right) and the counter (left).

To print a total, a counter "exit" is wired to the desired printer columns. On the plugboard, the printer columns are labeled print entries: 43 "alphamerical print entry" positions that can print alphabetical or numerical characters, followed by 45 "numerical print entry" positions that only print numbers. The diagram below shows four wires from counter 4C to print columns 1 through 4 (yellow), and six wires from counter 6C (red) to print columns 35 through 40.

Wiring a counter to a "print exit" causes the counter value to be printed.

The accounting machine contains 16 decimal counters in all. Four of them are 8-digit counters, named 8A, 8B, 8C and 8D. Four are 6-digit counters (6A to 6D), four are 4-digit counters (4A to 4D), and four are 2-digit counters (2A to 2D). In addition, two counters can be joined together to form a larger counter. There are also connections between counters for subtotals. For instance, counter 8A accumulates a per-employee subtotal. These subtotals are added to counter 8B to form the final total.

Another important operation is to compare two cards to see if they have the same id (and should be counted together) or if they have different ids (so a subtotal should be printed and the counters reset). A comparison is done by wiring two fields to the two "comparing entry" rows. If the fields are different, the "comparing exit" will trigger a signal. Since we want to compare each card with the next card, we get one field from the "second reading" and one from the "third reading"; the card we are processing will be at the third reading stage while the card behind it will be at the second reading stage. Finally, the comparison output is wired to the "program start (minor)" hub. This causes the accounting machine to start an additional cycle to print the subtotals (i.e. minor totals) and reset the counters. (There are also "intermediate" and "major" program start hubs, which provide two additional levels of totals.)

Columns 1-4 of the cards are compared to determine if subtotals should be printed.

On the diagram above, columns 1-4 from the second reading and from the third reading are wired to the comparing entry hubs. The four corresponding comparing exit hubs are wired together (gray) and connected to the minor (MI) program start hub (yellow wire to PRG START in upper right). The closeup of the plugboard below shows the wiring on the plugboard.

Columns 1-4 of the cards are compared to determine if subtotals should be printed.

Another interesting feature of the plugboard is conditional behavior, using "selectors". Connections can be switched based on a different signal, allowing behavior to change based on a comparison, or a panel switch. This plugboard changes behavior based on the "setup change 1" panel switch, one of the switches on the accounting machine above the panel. (You can think of this as the plugboard version of command-line options.) According to the label on the plugboard (below), this switch selects "year to date". On the board, this switch enables processing of one field, as well as switching between the constant 2 and 5 for addition to counter 2B. (The reason for this constant is a mystery to me.)

The label on the plugboard shows it computes tax deductions.9 "S/P" is presumably "summary punch". The setup 1 switch selects "year to date".

The wiring on the right side of the plugboard controls the counter behavior, such as accumulating subtotals versus final totals. It also wires some of the counters together to form larger counters. For instance, counters 2C and 4D are combined to form a single 6-digit counter. 8 I won't explain the counter control wiring here; the manuals15 explain how it works.

"Summary punching" is another interesting feature of the accounting machine. This lets you take a large file of cards and punch a smaller summary file. For the tax plugboard, one summary card is punched for each employee, with the totals for that employee. Thus, a card file with one record for each employee's pay period is reduced to a much smaller file with one card for the employee's yearly totals. This smaller card file can then be used for further processing.

IBM 403 accounting machine connected to a 519 summary punch. Courtesy Columbia University Computing History.

Summary punching is accomplished by connecting a summary punch machine (above right) to the accounting machine (left) through a thick cable. A hub on the plugboard is wired to enable summary punching, and another hub is wired to control when to punch a card. For the tax plugboard, a summary card is punched for each minor total with the wiring below. A separate plugboard on the summary punch machine controlled which columns were punched on the summary card.

Summary punch wiring on the IBM 403 plugboard. The summary punch control pickup (SP Control PU on the left) is wired to punch a summary card on a minor total. The summary punch switch (SP.SW) hubs are connected by the gray wire (lower left).

Inside the 403 Accounting Machine

Its amazing how much functionality these accounting machines could provide without the benefit of electronics, purely through clever electromechanical systems. Inside the accounting machine is a maze of motors, rotating shafts, cams and clutches, making it seem more like a car than a computer—it even contained an oil pump! With all these mechanical parts a 403 accounting machine weighed over a ton (2515 pounds / 1143 kg).

Inside an IBM 403 Accounting Machine, front view. From the 402/403 Field Engineering Manual, fig. 5.

On the plugboard, a wire is used to route a column of the card. How does a character on the card get sent across this wire? How does a counter perform addition? And how does the result get printed? The accounting machines use clever mechanisms, closely tied to the structure of a punched card, to perform these operations.

In modern terms, a character is encoded serially over a wire, by a single pulse whose timing depends on the position of the hole. These pulses start and stop the counters used to add values. These pulses also control the timing of the typebars that print the result. How these pulses are generated and how they electromechanically control the system will be described more below.

The 403's timing is based off the rotating shafts that drive the machine, rather than clock time. Each revolution of the shaft corresponds to a "card cycle", the reading and processing of one card. The fundamental timing unit is a rotation of 18°: this is the time between reading successive card holes, moving a typebar by one character, and rotation of a counter by one count. At 150 cards per minute, these values work out to approximately 400 milliseconds per card and 20 milliseconds per 18° step, remarkably fast for mechanical operations.

Reading cards

To understand the accounting machine, one must first understand how punched cards hold data. Punched cards hold 80 characters of data; each character is represented by the hole pattern in a column. The card below shows how numbers and the alphabet are punched; each character is printed at the top of the card with the corresponding punches in the column below. A digit is simply represented by a hole in the corresponding row, 0 through 9. (Note that numbers are stored in decimal, not binary.) To support alphanumeric data, two "zone" rows were added above the digit rows.10 A letter is represented by putting two holes in a column: a zone punch and a digit punch.11

An 80-column IBM punched card. Each column encodes a character (printed at top) by punching holes in the column. For a digit, a hole is punched in the row with the same number. A letter is encoded by adding a "zone punch" in one of the top three rows.

You might expect the accounting machine to read cards a column at a time, so one character gets processed at a time. But instead, cards are read "sideways", starting at the bottom. All 80 columns are read in parallel, one row at a time, starting with row 9 and ending with row 0 and then the zone rows. The accounting machine uses sets of 80 wire brushes to read a card, one for each column. If there is a hole, the brush makes contact with the energized metal roller underneath the card, completing a circuit and generating a pulse. Thus, each column will have a pulse corresponding to its hole, with the 9 pulse first, followed by 8 and so forth, ending with 0. Thus, each character is encoded serially, and each plugboard wire carries one of these serial signals, but all columns are processed in parallel.

Printing

Typebars in an IBM 402 accounting machine. Courtesy Columbia University Computing History.

The accounting machine's printing mechanism consists of 88 typebars;12 each vertical bar holds all the characters that can be printed. The typebars move vertically to line up the proper characters and then hammers13 hit the typebars into an inked ribbon to print the selected characters. Thus, the characters in a line of text are printed simultaneously.

The wires in the plugboard control what gets printed by stopping each rising typebar at the right time to select the desired character. The motion of the typebars is carefully timed to match the reading of a card, so the "3" row (for instance) of a card is read at the same time that the "3" on the typebar moves into position. If the brush's hub is wired to a column's print hub, this signal energizes a print magnet, releasing a "stop pawl" which meshes with a tooth on the typebar, stopping it with the "3" character in position to print. If a "2" is read instead, the brush reads the hole one time unit later; the typebar will have risen one more position, causing a "2" to be printed.

The printing mechanism consists of a complex arrangement of mechanical parts: cams, pawls, slides, springs and clutches, in combination with electromagnets to activate these parts at the right time. The mechanism can print 100 lines per minute, so the parts are flying around rapidly and require exact timing. The typebars move one position for every 18° rotation of the driveshaft, keeping them synchronized with card reading.

Counters

The heart of the accounting machine is the electromechanical counters that sum the values. Each digit in a counter is represented by a wheel that rotates to perform addition. The position of the wheel indicates the digit. For instance, to add 27 to a counter, the tens digit wheel is rotated two positions and the unit wheel is rotated seven positions. Thus, to add the value in a card field, the wheels must rotate an amount corresponding to the number punched in the card. The wheel starts rotating when a hole is read, rotates one position as each additional row is read, and stops reading at row 0. Since row 9 is read first and row 0 is last, the result is the counter rotates the number of positions indicated by the hole.

An electromechanical counter from the IBM 403 accounting machine performs addition on two digits by rotating the counter wheels.

The photo above shows a two-digit counter unit. The counter wheels are at the left. The start and stop coils cause the counter to start and stop rotating at the correct times by activating lever arms that control a clutch under the wheel. Carry is implemented by cams underneath the wheel that close electrical contacts. On the back of the board are the electrical contacts that read out the value stored in the counter; these are wired to the connector on the right.

Diagram of the electromechanical counter, indicating the key components. From the IBM 403 Field Manual.

The plugboard specifies which card columns are added to which counter digits. To add a field's value to a counter, a column's read brush is wired to the counter through the plugboard, so the card controls how much the counter rotates. This signal activates the counter's start coil, engaging the counter's clutch and starting the counter's rotation. At the 0 position, the stop coil disengages the clutch, stopping the counter. For instance, if the brush read a 7 from the card, the counter will rotate through seven positions before stopping, adding 7 to its value. If the brush read a 1, the counter will rotate by just one position. The reason this works is the synchronization between card movement and counter rotation; an 18° rotation corresponds to the card moving by one row as well as one count on the counter wheel. (A counter wheel has 20 positions spaced 18° apart. Counting by 10 rotates the wheel halfway.) Subtraction is performed by adding the complement.14

A carry from one position to the next is handled by a complex mechanism. You might expect that when one wheel rolls over from 9 to 0, it increments the higher wheel like an odometer, but that would be slow for multi-digit counters. (Keep in mind that the counters can add 150 numbers per minute, so they are spinning rapidly.) Instead, the counters use a mechanism similar to carry lookahead. If a wheel is at 9, an electrical contact closes, allowing a lower-order carry to be passed through to the higher wheel. If a wheel passes from 9 to 0, it closes a different electrical contact, generating a carry. After the "regular" addition, any necessary carries are generated in parallel and added in a single time step. Thus, something like 99999999+1 isn't delayed by a ripple carry; instead all digits get a carry in parallel.

Relays

The accounting machine is controlled by hundreds of relays, electromechanical switches that provide all the "control logic" for the system. The photo below shows the back of the accounting machine, filled with relays; more relays are on the end panel. To generate timing signals signals for the relays, switches were opened and closed by cams attached to the rotating shaft. Thus, everything in the system is timed from the rotating shaft.

The IBM 403 accounting machine is controlled by hundreds of relays, many of which are mounted in the back of the machine. Photo from the Field Engineering Manual, fig 81.

Conclusions

Punched card data processing is almost forgotten now, but it ruled data processing for almost a century. Even before computers existed, businesses used punched cards and tabulators for accounting. IBM's accounting machines were able to perform surprisingly complex tasks even though they were built from electromechanical components that seem primitive today. Accounting machines and plugboard programming remained popular into the 1960s, when businesses gradually switched to stored-program business computers such as the IBM 1401. Even so, IBM continued marketing accounting machines until 1976. Incredibly, one company in Texas still uses an IBM 402 accounting machine for their accounting today (details), illustrating the amazing longevity of punched card technology.

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Thanks to Carl Claunch (one of the Xerox Alto restoration co-conspirators) for providing the plugboard and documentation.

Notes and references

1. April 15 is traditionally tax day in the US, but if you don't have your taxes done yet, don't panic. In 2017, US tax day is April 18 due to the weekend and holiday. ↩

2. To support addition, tabulators used a module called an "accumulator" with rotating dials to hold decimal numbers. This accumulator gave its name to the accumulator register still used in microprocessors today. For example, Intel's x86 processors have a register called EAX, the EXtended Accumulator. ↩

3. The history of IBM's tabulating machines is described in IBM's Early Computers. Also see Columbia University's computing timeline. ↩

4. Another part of the unit record system is the card sorter, which rapidly sorts cards on a field, putting them in the proper order to be processed by an accounting machine. I discuss IBM card sorters in detail here. ↩

5. The 402 and 403 accounting machines were essentially the same except the 403 could print three-line addresses. In order to print three lines from one card, the 403 has three card reading stations instead of two. (That is, it read each card three times using three sets of 80 brushes). This feature is called MLP (multi-line printing) and is useful for printing addresses on invoices, for instance. An MLP card is indicated with a special punch: 8, 9 and (1, 2, 3 or 4) punched in a single column; the last digit controls the number of lines printed. ↩

6. I wouldn't be surprised if these accounting machines were technically Turing-complete due to their support for conditional operations, although it's unclear how to represent the tape. Perhaps storage could be implemented by punching a new deck of cards on each cycle through the machine. Of course this would be impractical for any real use. ↩

7. I suspect each card represents one employee pay stub and each field indicates a payroll deduction. However, there are alternative explanations for the plugboard. For instance, the id field could indicate a company division, and each card represents a subdivision. In this case, the accounting machine could be totaling the tax deductions for each division such as business expenses and depreciation. Or each card could represent one month. Since there are no variable names, it is speculation. ↩

8. The table below summarizes the program implemented by the plugboard, showing the mapping between input fields on the card and output fields on the printer.

   Card columns	Output columns	Subtotal counter	Total counter
   1-4	1-4	4C
   34-38	5-10	8D	4A/2D
   44-45	11-18	8A	8B
   61-66	19-26	6A	2C/4D
   67-71	27-32	6B	2A/4B
   28-33	35-40	6C	8C
   14-17		4D
   from switch		2B

   Columns 14-17 are summed but not printed. Presumably they are punched on the summary punch card. Columns 34-38 are only processed if the "setup change 1" switch is active. Counter 2B is controlled by the panel switch, adding either 2 or 5 each step. I can't figure out a reason for this; I assume the plugboard on the summary punch (which I don't have) does something useful with this value. ↩

9. The tax plugboard I'm examining was labeled with embossing tape which dates the labeling of the board to post 1958. The board could originally be older, or it could have been used into the 1960s. ↩

10. The row above "0" is called 11 or X, while the row above that is 12. For alphabetical characters, the "0" row is used as a zone instead of a digit. (This causes some complications in the accounting machine, such as a special mechanism to print a "numeric zero" versus a "zone zero".) ↩

11. This punch card code evolved into EBCDIC (Extended Binary Coded Decimal Interchange Code), the encoding used by IBM computers in place of ASCII. Many of the strange characteristic of EBCDIC, such as the alphabet not being entirely sequential, are due to its roots in punched cards. ↩

12. The accounting machine has typebars on the left that print alphanumerics and typebars on the right that just print digits. For alphanumeric printing, the digit signal moves the typebar in steps of four, while the zone signal moves the typebar 0 through 3 steps. Thus, an alphanumeric character can be printed. Typebars with special characters could also be installed, to print $, @, - or %. ↩

13. You might expect that to print each line, all the hammers hit the typebars, but it's more complex than that. First, each hammer has a mechanical "hammerlock" control, which can enable the hammer, disable the hammer, or put the hammerlocked hammers under program control. Thus, part of the line may be printed or suppressed based on the data. In addition, the hammers also have mechanical "hammersplit" levers which when raised cause leading zeros in a field to be suppressed. This allows the value "000123" to be printed as "   123" for instance. ↩

14. Subtraction uses 9's complement addition. That is, subtracting a digit n is done by adding 9-n. This is accomplished mechanically by starting the counter's rotation at position 9 and stopping when a hole is read. For example, if the hole is at position 7, the counter will increment by two positions. There are a few complications with 9's-complement subtraction. The answer is off by one, but an "end-around carry" adds 1 to yield the correct result. Negative numbers require special handling to be printed properly using the "net balance method" or the "balance selection" method; see the 403 manual if you care about the details. The numeric typebars include a "CR" symbol, which indicates negative numbers as a "credit". On a punch card, negative numbers are typically indicated with an X-punch (i.e. a zone punch in row 11) over the value. ↩

15. IBM's accounting machine manuals are available on Bitsavers. The operation of the IBM accounting machines is discussed in detail in: IBM 402, 403 and 419 Accounting Machines: Manual of Operation. For a thorough discussion of how the machine works internally, see IBM 402, 403, 419 Field Engineering Manual of Instruction. For an overview of how plugboard wiring for IBM's works, see IBM Functional Wiring Principles. ↩