Inside the digital clock from a Soyuz spacecraft

We recently obtained a clock that flew on a Soyuz space mission.1 The clock, manufactured in 1984, contains over 100 integrated circuits on ten circuit boards. Why is the clock so complicated? In this blog post, I examine the clock's circuitry and explain why so many chips were needed. The clock also provides a glimpse into the little-known world of Soviet aerospace electronics and how it compares to American technology.

"Onboard space clock" from a Soyuz mission. The clock provides the time, an alarm, and a stopwatch.

"Onboard space clock" from a Soyuz mission. The clock provides the time, an alarm, and a stopwatch.

The Soyuz series of spacecraft was designed for the Soviet space program as part of the race to the Moon. Soyuz first flew in 1966 and has made more than 140 flights over the past 50 years. The spacecraft (below) consists of three parts. The round section on the left is the orbital or habitation module, holding cargo, equipment, and living space. The descent module in the middle is the only part that returns to Earth; the astronauts are seated in the descent module during launch and reentry. Finally, the service module on the right has the main engine, solar panels, and other systems.

Soyuz TMA-7 spacecraft departing from the International Space Station, 2006. Photo from NASA.

Soyuz TMA-7 spacecraft departing from the International Space Station, 2006. Photo from NASA.

The descent module contains the spacecraft's control panel (below).2 Note the digital clock in the upper left. Early Soyuz spacecraft used an analog clock, but from 1996 to 2002, the spacecraft used a digital clock.3 The digital clock was also used in the Mir space station. The clock was eliminated from later Soyuz spacecraft, which used two computer screens on the control panel in place of the earlier controls.

Control panel from a Soyuz spacecraft. The digital clock is in the upper left of the panel. The screen in the middle is a TV monitor. Photo from Stanislav Kozlovskiy, CC BY-SA 4.0.

Control panel from a Soyuz spacecraft. The digital clock is in the upper left of the panel. The screen in the middle is a TV monitor. Photo from Stanislav Kozlovskiy, CC BY-SA 4.0.

A closer look at the clock

The diagram below shows the clock's labels translated into English. The clock has three functions: the time, an alarm, and a stopwatch. The "Clock of Current Time"5 mode shows the current Moscow time on the six upper LED digits, while "Announcement" shows the alarm time. The alarm can be set to a particular time; at that time, the clock triggers a relay activating an external circuit in the spacecraft.4 The clock is set using the "Correction" mode; digits are incremented using the "Enter" button. The lower half of the unit is the stopwatch; the bottom four LEDs display elapsed minutes and seconds. The lower pushbutton stops, starts, or resets the stopwatch.6 Finally, the power switch at the right turns the clock on.

Front of the clock. The red text is the translation of the Russian labels into English.

Front of the clock. The red text is the translation of the Russian labels into English.

We wanted to see what was inside the clock, of course, so Marc unscrewed the cover and removed it from the clock. This revealed a dense stack of circuit boards inside. The clock was much more complex than I expected, with ten circuit boards crammed full of surface-mount ICs and other components. The components are mounted on two-layer printed-circuit boards, a common construction technique. The boards use a mixture of through-hole components and surface-mount components. That is, components such as resistors and capacitors were mounted by inserting their leads through holes in the boards. The surface-mount integrated circuits, on the other hand, were soldered to pads on top of the board. This is more advanced than 1984-era American consumer electronics, which typically used larger through-hole integrated circuits and didn't move to surface-mount ICs until the late 1980s. (American aerospace computers, in contrast, had used surface-mount ICs since the 1960s.)

Space clock from Soyuz with the cover removed.

Space clock from Soyuz with the cover removed.

One interesting feature of the clock is that the boards are connected by individual wires that are bundled into wiring harnesses (below). (I expected the boards to plug into a backplane, or be connected by ribbon cables.) The boards have rows of pins along the sides, with wires soldered to these pins. These wires were gathered into bundles, wrapped in plastic, and then carefully laced into wiring harnesses that were tied to the boards.

The clock has point-to-point wires, wrapped into neat harnesses.

The clock has point-to-point wires, wrapped into neat harnesses.

At first, we thought that further disassembly of the clock would be impossible without unsoldering all the wires, but then we realized that the wiring harnesses were designed so the boards could be opened like a book (see below). This allowed us to examine the boards more closely. Inconveniently, some pairs of boards were soldered together at the front by short wires, so we couldn't see both sides of these boards.

The wiring bundles are arranged so the boards can swing apart.

The wiring bundles are arranged so the boards can swing apart.

In the photo above, you can see the numerous integrated circuits in the clock. These are mostly 14-pin "flat pack" integrated circuits in metal packages, unlike contemporary American integrated circuits which were usually packaged in black epoxy. There are also some 16-pin integrated circuits, encased in pink ceramic.

The circuitry inside

The next step was to examine the circuitry in more detail, which I'll discuss starting at the back of the clock. A 19-pin connector7 linked the clock to the rest of the spacecraft. The spacecraft provided the clock with 28 volts through this connector, as well as external timing pulses and stopwatch control signals. The clock could signal the spacecraft through relay contacts when the alarm time was reached.

This 19-pin connector interfaces the clock to the spacecraft.

This 19-pin connector interfaces the clock to the spacecraft.

The two circuit boards at the back of the clock are the power supply, which was more complex than I expected. The first board (below) is a switching power supply that converts the spacecraft's 28-volt power to the 5 volts required by the integrated circuits. The round ceramic components are inductors, ranging from simple coils to complex 16-pin inductors. The control circuitry includes two op amps in metal can packages. Two other packages that look like integrated circuits each hold four transistors. Next to them, a bullet-shaped Zener diode sets the output voltage level. The large round switching power transistor is visible in the middle of the board. You might expect the power supply to be a simple buck converter. However, the power supply uses a more complicated design to provide electrical isolation between the spacecraft and the clock. I'm not sure, though, why isolation was necessary.8

Board 1 implements a switching power supply to produce 5 volts for the clock.

Board 1 implements a switching power supply to produce 5 volts for the clock.

Many of the components in the power supply look different from American components. While American resistors are usually labeled with colored bands, the Soviet resistors are green cylinders with their values printed on them. The Soviet diodes have orange rectangular packages (below), unlike the usual cylindrical American diodes. The power transistor in the middle of the board is round, lacking the metal flanges of American power transistors in "TO-3" packages. I don't think the Soviet packaging is better or worse, but it's interesting to see how components from the two countries diverged.

The power supply uses 1 amp diodes in rectangular orange packages. The "OC" indicates a higher-quality military part.

The power supply uses 1 amp diodes in rectangular orange packages. The "OC" indicates a higher-quality military part.

The second board is also part of the power supply, but is much simpler. It has inductors and capacitors to filter the power, as well as a linear voltage regulator chip (pink) to produce 15 volts for the op amp ICs in the first board. The voltage regulator chip has two large metal tabs on the bottom that were soldered to the circuit board to dissipate heat. Strangely, the board has three large holes in the right side. The obvious explanation would be that these holes made room for tall components, a situation that arises on another board. However, there are no components that fit the holes on this board. Thus, I suspect this board was originally designed for a different device and reused in the clock.

Power supply board 2 is half-empty, with the right half apparently acting as a heat sink.

Power supply board 2 is half-empty, with the right half apparently acting as a heat sink.

The remaining boards are filled with digital logic integrated circuits. Board 3 (below) and board 5 (which is similar) implement the current time and alarm time functions. Each board contains six BCD counter chips for the six digits (hours, minutes, and seconds).9 In addition, each digit counter requires a logic chip to control when it is incremented and another chip to control when it is reset, depending on whether the clock is being set or is running. (This is one reason why so many chips are required.) The pink chip on the board controls which digit is modified when setting the clock.10

Board 3 is filled with digital logic integrated circuits. Pins on either side connect the board to the wiring harnesses.

Board 3 is filled with digital logic integrated circuits. Pins on either side connect the board to the wiring harnesses.

Board 4 (below) has two functions. First, it controls whether the clock displays the current time or the alarm time. This is implemented with a selection chip for each digit. Second, the board signals the spacecraft when the current time reaches the alarm time. This is implemented with multiple chips to step through each digit, compare the times, and determine if they match. Thus, even though the functions of this board seem simple, they require a whole board of chips. The connections at the bottom of the board link board 4 to board 5. The board is connected to board 3 through the wiring harness.

Board 4 selects between the current time and the alarm time. It also compares the two values to determine when the alarm time has been reached.

Board 4 selects between the current time and the alarm time. It also compares the two values to determine when the alarm time has been reached.

Some of the boards have more circuitry than just digital logic. For instance, boards 6 and 7 have pulse transformers to electrically isolate the control signals fed into the clock through the 19-pin connector. (In modern circuits, this role would be performed by an optoisolator.) These transformers look a bit like mushrooms or miniature water towers, and can be seen in the photo below. Board 7 also has a quartz crystal, the metal rectangle below.11

Board 7 has a 1 MHz crystal that provides the timing signals for the clock. It also has three round pulse transformers that isolate the control signals from the spacecraft.

Board 7 has a 1 MHz crystal that provides the timing signals for the clock. It also has three round pulse transformers that isolate the control signals from the spacecraft.

The two functions of board 7 (below) are to generate the clock's timing pulses and to implement the stopwatch. The quartz crystal generates accurate 1 megahertz pulses. These pulses are reduced to one-second pulses by six BCD counters; each counter chip divides the frequency by 10. These timing pulses are used by the rest of the clock. To implement the stopwatch, the board has four BCD counters for the four digits. It also has control logic to start, stop, and reset the stopwatch. The three pulse transformers allow the spacecraft to control the stopwatch when certain events happen. Additional chips handle these mode changes.

Board 7 contains the stopwatch circuitry, as well as the quartz crystal that generates timings for the whole clock. Wires along the front connect the board to Board 6.

Board 7 contains the stopwatch circuitry, as well as the quartz crystal that generates timings for the whole clock. Wires along the front connect the board to Board 6.

Boards eight and nine drive the LED displays. Each LED digit requires a chip to illuminates the appropriate segments of the 7-segment LED based on the BCD (binary-coded decimal) value. These BCD-to-7-segment driver chips are the pink 16-pin chips on the board.12 Since the clock displays 10 digits in total, 10 driver chips are used. Eight driver chips are on board 8, while board 9 has two chips along with numerous current-limiting resistors for the LEDs. The switches to control the clock are also visible in the photo below.

Board 8 is an LED driver board holding eight 7-segment driver chips. Board 9 (underneath) has two more driver chips and many resistors.

Board 8 is an LED driver board holding eight 7-segment driver chips. Board 9 (underneath) has two more driver chips and many resistors.

Finally, board 10 (below) holds the ten LED digits. Each digit consists of a seven-segment LED, along with a comma. I think one of the commas is wired up to indicate something; we'll find out what when we power up the clock.

Board 10 holds the ten LED digits. Photo from Marc Verdiell.

Board 10 holds the ten LED digits. Photo from Marc Verdiell.

Soviet integrated circuits

Next, I'll discuss the integrated circuits used in the clock. The clock is built mostly from TTL integrated circuits, a type of digital logic that was popular in the 1970s through the 1990s. (If you've done hobbyist digital electronics, you probably know the 7400-series of TTL chips.) TTL chips were fast, inexpensive and reliable. Their main drawback, however, was that a TTL chip didn't contain much functionality. A basic TTL chip contained just a few logic gates, such as 4 NAND gates or 6 inverters, while a more complex TTL chip implemented a functional unit such as a 4-bit counter. Eventually, TTL lost out to CMOS chips (the chips in modern computers), which use much less power and are much denser.

Because each chip in the Soyuz clock didn't do very much, the clock required many boards of chips to perform its functions. For example, each digit of the clock requires a counter chip, as well as a couple of logic chips to increment and clear that digit as needed, and a chip to drive the associated 7-segment LED display. Since the clock displays 10 digits, that's 40 chips already. Additional chips handle the buttons and switches, implement the alarm, keep track of the stopwatch state, run the oscillator, and so forth, pushing the total to over 100 chips.

One nice thing about Soviet ICs is that the part numbers are assigned according to a rational system, unlike the essentially random numbering of American integrated circuits.13 Two letters in the part number indicate the function of the chip, such as a logic gate, counter, flip flop, or decoder. For example, the IC below is labeled "Δ134 ΛБ2A". The series number, 134, indicates the chip is a low-power TTL chip. The "Л" (L) indicates a logic chip (Логические), with "ЛБ" indicating NAND/NOR logic gates. Finally, "2" indicates a specific chip in the ЛБ category. (The 134ЛБ2 chip's functionality is two 4-input NAND gates and an inverter, a chip that doesn't have an American counterpart.) 14

Two integrated circuits inside the clock.

Two integrated circuits inside the clock.

The logos on the integrated circuits reveal that they were manufactured by a variety of companies. Some of the chips in the clock are shown below, along with the name of the manufacturer and its English translation. More information on Soviet semiconductor logos can be found here and here.

By looking up the logo on each chip, the manufacturer can be determined.

By looking up the logo on each chip, the manufacturer can be determined.

Comparison with US technology

How does the Soyuz clock compare with US technology? When I first looked at the clock I would have guessed it was manufactured in 1969, not 1984, based on the construction and the large number of simple flat-pack chips. In comparison, American technology in 1984 produced the IBM PC/AT and the Apple Macintosh. It seemed absurd for the clock to use boards full of TTL chips a decade after the US had produced single-chip digital watches.16 However, the comparison turned out to be not so simple.

To compare the Soyuz clock with contemporary 1980s American space electronics, I looked at a board from the Space Shuttle's AP-101S computer.17 The photo below shows circuitry from the Soyuz clock (left) and the Shuttle computer (right). Although the Shuttle computer is technologically more advanced, the gap was smaller than I expected. Both systems were built from TTL chips, although the Shuttle computer used a faster generation of chips. Many Shuttle chips are slightly more complex; note the larger 20-pin chips at the top of the board. The large white chip is significantly more complex; it is an AMD Am2960 memory error correction chip. The Shuttle's printed-circuit board is more advanced, with multiple layers rather than two layers, allowing the chips to be packed 50% more densely. At the time, the USSR was estimated to be about 8 to 9 years behind the West in integrated circuit technology;15 this is in line with the differences I see between the two boards.

The Soyuz clock board (left) and Space Shuttle computer board (right), to the same scale. Both use surface-mount TTL chips.

The Soyuz clock board (left) and Space Shuttle computer board (right), to the same scale. Both use surface-mount TTL chips.

What surprised me, though, was the similarities between the Shuttle computer and the Soviet clock. I expected the Shuttle computer to use 1980s microprocessors and be a generation ahead of the Soyuz clock, but instead the two systems both use TTL technology, and in many cases chips with almost identical functionality. For example, both boards use chips that implement four NAND gates. (See if you can find the 134ΛБ1A chip on the left and the 54F00 on the right.)

Conclusion

Why does the Soyuz clock contain over 100 chips instead of being implemented with a single clock chip? Soviet integrated circuit technology was about 8 years behind American technology and TTL chips were a reasonable choice at the time, even in the US. Since each TTL chip doesn't do very much, it takes boards full of chips to implement even something simple like a clock.

The next step will be to power up the clock and see the clock in operation. I've been studying the power supply so we can make this happen. I plan to write more about the power supply and other parts of the clock, so follow me @kenshirriff for details. also have an RSS feed. Until then, you can watch Marc's video showing the disassembly of the space clock:

Notes and References

  1. CuriousMarc obtained the clock from an auction and it was advertised as flown to space, but I don't know which mission it was flown on. The date codes on the components inside the clock are mostly from 1983, with one from 1984, so the clock was probably manufactured in 1984. The Russian name for the clock is "Бортовые Часы Космические" (Onboard Space Clock), which is abbreviated as "БЧК". 

  2. The photo of the Soyuz console was mislabeled as from Soyuz 7K-VI. However, that mission was in the 1960s and the Soyuz-7K console was much different. A photo of the Soyuz-7K console is in this Russian article

  3. The digital clock was used in the Soyuz-TM version of the spacecraft. This version of the console was known as Neptune (Нептун). For details on Soyuz consoles, see The Integrated Information Display System for the Soyuz-TMA. Two Russian documents are this and (this. The analog clock can be seen in a Scott Manley video here and in some photos by Steve Jurvetson. 

  4. Most of the description of how the clock works is based on my reverse engineering, so I don't guarantee that everything in this post is accurate. When we power up the clock, I'll find out what I got wrong :-) 

  5. The clock has the label "ЧТВ", which is an abbreviation for "Часы Текущего Времени". The Soyuz Crew Ops Manual translates this as "Clock of Current Time". 

  6. The Soyuz Crew Ops Manual has some information on the clock on page 35. According to the manual, the stopwatch is controlled automatically during the propulsion system engine burn timing, to measure the time between the Engine Fire command and the Engine Cut Off command. It also automatically measures the time during descent until contact. 

  7. The 19-pin connector was a standard Soviet military connector of type RS19TV (РС19ТВ in Cyrillic). I was able to find a matching connector on eBay, which we will use for powering the clock. 

  8. Cell-phone chargers, for instance, use isolated power supplies for safety, to protect the user from the dangerous 120-volt line voltage. The clock, however, is powered with 28 volts, so there's no obvious reason for electrical isolation. (The Apollo Guidance Computer's power supply, for example, used a non-isolated switching power supply.) 

  9. The clock uses a BCD counter chip for each digit with some exceptions. The top hours digit only goes to "2" (for a 24-hour clock), so two flip flops are used instead of a counter. The top digit for minutes and seconds needs to roll over at 6 (i.e. 60 seconds/minutes), so the clock uses a divide-by-12 chip similar to the 7492 chip. (The chip can be configured to roll over at 6 rather than 12.) 

  10. The pink chip on board 3 is a К134ИД6 decimal decoder, which selects one of 10 outputs based on the 4-bit BCD value fed into it. (The part number ИД indicates a decoder, Дешифраторы.) This chip is a copy of the American 74L42 chip. For some reason, the 16-pin integrated circuits are in pink ceramic packages, while the more common 14-pin integrated circuits are in metal packages. 

  11. The Soyuz Crew Ops Manual (page 35) specifies the clock's accuracy as 30 seconds per day, which isn't very good. In comparison, a low-cost Timex quartz watch from the early 1970s was accurate within 15 seconds per month. According to the manual, the clock could be synchronized to external time pulses. During launch/injection and autonomous orbital flight phases, the clock was synchronized to the Program-Timing Control Equipment (АПВУ). It could also be synchronized to the TV unit (KЛ110). 

  12. LED displays often use multiplexing, where one driver chip is shared across all the digits and the display rapidly cycles through the digits. This reduces the number of chips and resistors required. I'm not sure why the clock uses separate drivers instead of multiplexing. 

  13. For more information on Soviet integrated circuits, including the ones used in the clock, see the databook Интегральные микросхемы и их зарубежные аналоги (Integrated circuits and their foreign counterparts). 

  14. The Soviet IC designation system is described in detail on Wikipedia. There are a few complications that make a chip's designation different from the labels printed on the chip. Because Л and П (Cyrillic L and P) look similar on small chips, the chip labels use Λ (Greek L) in place of Л (Cyrillic L). The Greek D (Δ) may replace Cyrillic D (Д) to avoid confusion with Cyrillic А. Moreover, names for commercial chips start with K, unlike the military chips used in the clock. Thus, a chip labeled "Δ134 ΛБ2A" appears in databooks and on the web under the name "К134ЛБ2". 

  15. Two CIA reports (1974 and 1986) provide information on the lag between Soviet IC technology and Western technology. 

  16. US manufacturers implemented clocks on a single chip in the early 1970s. Mostek introduced a single-chip digital clock chip in 1972, the Mostek MM5017. In 1974, Intel introduced a watch using a low-power CMOS chip, the Intel 5810 In other words, the Soyuz clock could (roughly) have been replaced with a single chip a decade earlier. 

  17. The AP-101S computer in the Space Shuttle was part of IBM's System/4π line of avionics computers. This 64-pound computer was built from TTL integrated circuits, using the 74F00 series (Fairchild's FAST line) for improved performance. (Its memory, however, was built from high-capacity CMOS chips.) The AP-101S computer was an updated version of the AP-101B used in the earlier Space Shuttle flights. (See The new AP101S general-purpose computer (GPC) for the space shuttle and Space Shuttle Avionics Upgrade.)

    At first, it surprised me that they designed both Shuttle computers from low-complexity TTL chips, but it made sense when the design of the earlier AP-101B computer started in 1972. Back in the 1970s, minicomputers were commonly built from TTL chips because microprocessors were new and much slower than TTL. The first Shuttle computer achieved a speed of 0.42 MIPS. This performance was respectable in 1972 but poor by 1981, when the Shuttle first flew.

    To improve performance, a redesign of the computer started in 1982. The updated AP-101S computer stuck with TTL, so its performance improved only moderately, to 1.27 MIPS, slightly slower than the Motorola 68010 (1982) which ran at 2.4 MIPS. Unfortunately, the gap between TTL computers and microcomputers got exponentially worse, following Moore's law. By 1991, when the AP-101S first flew, the Motorola 68040 ran at 44 MIPS. And by the end of the Shuttle program in 2011, the Intel Core i7 processor ran at 100,000 MIPS, many orders of magnitude faster than the Shuttle computer.

    So why did the Space Shuttle use mostly-obsolete TTL technology in the 1980s redesign? One reason was backward compatibility. Since the first Shuttle computer used the proprietary IBM 4π architecture, it couldn't be replaced by an off-the-shelf microprocessor. Reliability was another motivation for TTL. Commerical microprocessors weren't designed for the reliability needs of space systems and lacked features such as radiation resistance and parity-protected caches. Finally, the aerospace development cycle is very long; although the Shuttle computer redesign started in 1982, the computer wasn't used on a flight until 1991 and remained in use until 2011. The point is that there were reasons to build aerospace systems from TTL, even though microprocessors were much faster, more compact, and lower power.