Apollo Comms Part 14: Sending Commands to the Spacecraft

Apollo Comms Part 14: Sending Commands to the Spacecraft

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This episode has been made possible thanks  to the sponsorships of PCBWay, ElectroRent   and Keysight Technologies. Greeting earthlings. If you follow the channel,  you know that we love all things Apollo,   and that during our last visit to Steve  Jurvetson’s amazing space collection,   we were given the opportunity to take two  holy boxes of Apollo electronics to our lab. These are the boxes that brought you voice,  data and live TV from the moon, and should   be early masterpieces of microwave electronics,  the blackest of black arts in analog electronics. Our spacecraft transponder  is now fully up and running,   and we have turned our attention  to the ground part of the link. In episode 12, we succeeded in recreating the  ground transmission infrastructure with modern   Keysight equipment, gracioulsy  loaned to us by ElectroRent.  

And a bit earlier, in episode 9, we had worked  on powering up a completely undocumented piece   of Apollo ground test equipment: the  Up-data Link Confidence Test Set.   We returned it to partial life, thanks to Ken  Shirriff’s incredible reverse engineering. This test box should enable us to send  commands to the spacecraft from the ground.  

But although we eventually succeeded in making  it read in a tape after a lot of effort,   we still don’t understand it completely. And  worse, it is missing some important parts. In this episode, we’ll attempt to  complete the bring up of the box,   and see if we can temporarily replace  the missing parts with more HP equipment.   We hope we can then tie it to our Keysight  ground installation. And since this all   happens at audio frequencies, we might even be  able to hear what these commands sound like. Hey Mr. Ken, can you explain what you're doing?  [Ken] So my latest project is, I'm trying to  

figure out how this box is wired up. It has 25  boards that plug in, and then a wire-wrapped   to back plane that's just a tangle of wiring. So  it's hard to figure out what is connected to what. [Marc] All the cables are the same color. And they all leak into each other.  So we tried with a wire tone tester,   and that didn't do anything. [Ken] And  we tried the current tracer as well,   that didn't work. [Marc] Yeah. [Ken] The  multimeter, I could get a lot of the wiring,  

but it's very, very slow. And then down here,  you can't even access the pins, there's so much   wiring. [Marc] Yeah, and the pins are into the  connectors, it's really hard to get to them. [Ken] So I made an automated system to  help me figure it out. What I do is,  

I have two boards here. I plug them into two  slots, and then I run through all the pins,   activating each pin in sequence, and then  reading to see what it's connected to. And so,   that gives me the wiring between these two slots.  And so, then I put the two boards into every pair   of slots. So that's 300 pairs altogether. And then  it will give me the entire wiring for the the box.

[Marc] And we wanted to give our traditional  shout-out for our sponsor PCBWay. They   make all of our boards, from the rather  simple one here, to the very complicated,   and they have been our sponsor since the channel  was minuscule, so we thank them very much. [Ken] These boards are very simple. There's  three 16-bit general purpose I/O chips   to drive the pins here, either reading or writing.  And it's controlled over I2C, a simple serial   protocol. It's not really Ethernet. I'm  just using Ethernet cables because they're   convenient, I had them around. And  it's driven by the Teensy here,  

an Arduino-like microcontroller, that just sends  serial signals to the two boards to activate each   pin and read the results. And then the results are  sent to my laptop. And I record it in a data file,   where I can then apply graph algorithms to figure  out what the nets are, what is connected to what. So I'll plug the pair of boards  into slots 17 and 18 here.   The boards pop in. And I tell the program that  I'm using slot 17 and 18. And there, it scanned   all the approximately 5000 potential wire pairs  between those two boards in under a second.

So now I've moved it over to slot  19. I run the test on slot 19. [Marc] Already done, eh? [Ken] Yeah, it's under a second, it's very  fast! I put it in slot 20. So, inconveniently,   half the slots are upside down, so I have to keep  flipping the board over. [Marc] Really? [Ken] In   the actual modules they have two boards sandwiched  together. [Marc] Oh, I see! [Ken] So they're  

inverted relative to each other. [Marc] So it's  pairs of connectors. [Ken] Yeah. And so I just   keep going through all 300 pairs of slots. [Marc]  You have made a reverse engineering machine! [Much later...]

So we have everybody: we have Ken, Mike and Eric.  And we are all admiring our up data... No. Yes.   Uplink? Updata box? What is it? [Ken] Updata  test box. [Marc] Updata test box, which,   Ken, you have sort of almost finished your   reverse engineering of it? [Ken] So I've spent way too much time   reverse engineering this. And basically, what  it does is, it reads a message off paper tape,   loads it into the digits here. And then it sends  the message, encoded using phase shift keying to  

the real updata box. Which sends messages then to  other parts of the spacecraft such as the Apollo   Guidance Computer. This box then reads reads  back in the message from the external boxes,   and verifies that it came through  the whole system correctly. [Marc] Last time, we just had this box barely  working, and then you made a contraption to read   the bus, so you could get all the connections  on the bus? [Ken] Yeah, so I made some boards   that would probe the backplane, which is a  massive point-to-point wiring. So I could   figure out what boards, how they're connected,  and try to piece together what they're doing. [Marc] And you actually did! [Ken] So I made this diagram. So each box represents one of the  

boards here. So, the big picture is, message  comes in from the paper tape reader, gets loaded   into the 13 digits here. These digits are set up  as a shift register, so it can then rotate through   all the digits, send the digits to other parts  of the system. And they get sent out externally.   And then, the inputs come in, and the inputs get  compared to what the box thinks they should be.  

And then the lights will light up with  with red or green, whatever worked or not. [Marc] Let's load one tape on the indicators.  I have one ready. 'Program Start',   that should work. [Eric] Is there  another code for the end of the tape,   or is it just that bit to start? [Marc] No,  there's just that bit for the start I could find.   There's another control bit that makes you ignore  the stuff and that's probably so you can...

Oh, hop, hop, hop! 'Single'!   All right, there you go. So, it was  reading commands and sending them.   But we loaded zero, one, two, three, four,  five, six, seven - no eight - zero, one,   two three, four, because you run out of numbers. So the only other control bit is   to ignore the input. It will just skip right over  it. [Ken] If you mistype something, you can just  

cancel that that and keep going without having  to redo your whole tape. [Ken] That makes sense,   if they were punching these tapes by  hand, you know, it would have been a   pretty error prone process. [Marc] Yeah. [Ken]  Marc, was a pretty error-prone? [Marc] Yes! But we're doing pretty good here. We have no tape  errors, so we got that to read properly. [Ken] So   we read 13 digits off the tape. The first three  are a message number which is not sent. It's only   used for knowing what test you're doing. The next  digit is the vehicle address. So, you can send a  

message to the command module, the lunar module,  and make sure that the right spacecraft gets the   message. The next digit is what system on the  spacecraft you want to control. And then the   next is the message which depends on the system  you're sending it to. The message can turn a relay   on or off, it can send a command to the Apollo  Guidance Computer, it can update the clock on the   spacecraft. So, these last digits are the ones  where it actually does something interesting. [Marc] Most of our knowledge on how these commands  work come from this drawing from the CSM 114   Command Module Systems Handbook, which happens  to be the command module for Apollo 17.   It’s a quite complicated drawing, so let’s  spend some time on it with some elevator music. Let’s start at the right side of the  drawing, which shows the commands structure.

There are four of commands:  CTE commands, for the Central Timing Equipment CMC commands, for the Apollo Guidance Computer  Test messages, for checking bits and RTC commands, for Real Time Control The CTE is mostly a clock. There  is a control line for reset,   and one line each for the  seconds, minutes, hours and days.   It’s very simple. Each time you put a pulse on  the line, you increment the corresponding digit.

Let’s take a look at at  CTE command bits in detail. The first 3 bits are called VA, for the  Vehicle Address. For the command module,   it is always hardwired to 4. The second digit, also 3 bits,  

is the System Address, of which only 4  are used. For CTE, it’s hard wired to 4.  So a CTE command starts by 44. The next values are in bunches of 6 bits each.  They are the seconds, minutes, hours and days.   Basically, after receiving this command,  the hardware will reset the CTE to zero,   and pulse each set line to make them  advance. So, that’s how you reset the clock.

Let’s look at the CMC messages, which is  the name of the AGC in the command module.   The AGC is connected to the message system   via only two lines, one for sending  ones, and one for sending zeroes. But note that there are two switches in the way.  They both say block or accept.  One is on the main command panel.

And the other one is in the lower bay,  next to the star alignment equipment. These are marked block and accept, and unless  the astronauts put both switches on accept,   access to the computer from the ground  is not allowed. So, you can see, they were   really worried about possible interference,  in particular from Russian interference.   So, before each access to the computer,  the ground would have to call the astronauts   and ask them to flip the switches, and  you can actually hear that on the loop. Let’s see how the computer commands are formed.  A computer command always begins with 43. 

Then, 5 command bits are  transmitted 3 times: in normal form,   in bit inverted form, and in normal form  again. This is obviously for error checking. The 5 bits in question represent a  single DSKY key press code. So basically,   the ground is simply remotely  pressing on the keys of the DSKY,   one at a time. Although very rudimentary, this  is deceptively powerful. The ground can do   everything the astronauts can do with the AGC,  which is pretty much flying the whole spacecraft. You can reorient it, fire the engine, load burn  data, check your trajectory.  

The ground can also read the computer results  via the much faster telemetry downlink.   For example, they could do a full  memory core dump if they wanted to. Let’s now look at the test message system, which  allows to test each bit sent to the system.   The test results are once again sent via the  telemetry downlink. One test message sends zero   bits, and the other sends one bits. You can  then rotate them and check them one by one.   The system gives you the results via the  telemetry, and there are only a few messages:  - standby, if nothing is decoded - validity, if any error-free message is received  - Results of the check of either the  ones and zeroes and off if system is off.

Last but not least are the Real Time Control  messages, which flip a bunch of relays.  Here are the coils of the relays  controlled by this message.  An RTC command begins with 46.  The rest of the bits indicates  which relay coil is to be energized.  There are also reset messages that flip a whole  bunch of relays at a time, called salvo reset.   They start with 45. They are used to put the  spacecraft back into a default configuration.

The RTC messages are actually  spelled out over here. We can see for example that 4600 will turn relay  00 off, which will turn the Abort Light A off.   4601 will turn the abort light on. And here are the salvo resets.  Salvo message 4530 followed by  

4532 will flip this whole bunch of relays. Now let’s see how these relay  contacts are implemented.  Here are the contacts connected to  coils 42, 43, 46, and 47. You can see  

that they control our S-Band amplifier: Off, Low Power, High Power, or Reset. But wait, didn’t the astronauts also  have a switch that did the same thing? And yes, they did! Here is the astronaut  switch that does the same thing!   So, who wins? The ground or the astronauts? Well,  if you follow the wiring, it’s mostly the ground.   You can see that the astronaut switch only  does what it says on it, if the RTC relays   are in the Reset position. Any other RTC  command on these relays, and the ground wins,  

it does not matter which position the  astronauts have set their own switch. I have not looked at all the wiring, some  get quite complicated, like this one,   which is the SBAND AUX control but it appears that ground wins most of the time. So you can see how the up-data commands were  done. You get all you need with relatively   unsophisticated electronics and a few simple messages. The simple things are done directly via relays,  

and the complicated ones are done via  remote pressing the keys on the AGC. This whole Apollo thing is really  about cleverly managing complexity. And, so, we actually got enough of it to work,  that we can do the rest with some crutches.   So, we got the bits coming out, the  bits and the sub-bits. Actually,   we have to explain this. Oh, that's tough. Do  you want to explain the bits and the sub bits? [Ken] I'd be delighted to! [laughter]  So, the message, of course, is sent in binary.  So each octal digit turns into three bits.  

To make this reliable, they use something they  call sub-bits, where each bit is expanded into   five bits, which are the sub-bits. So a zero  turns into one sequence of five bits, a one turns   into another sequence of five bits. And if any of  those bits are corrupted, you can detect an error.   So it makes it much less error-prone, because  you want to have reliability for the spacecraft. They also use a different sub-bit  encoding for the vehicle address,   so they can tell if it's the start of a message,  versus if they pick up in the middle of a message.  

And so, that is another way they get reliability. [Marc] For the for the framing, actually. They   need a framing mark. So they need a code that  only appears at the beginning of the frame,   that's different from everything else.  [Ken] And that's done by using the different   sub-bit encoding. [Marc] That's one of the  methods to do it. [Ken] And then for testing,   they have the sub-bit spoil switch,  which will spoil a few of the sub-bits.   So you can test if the error checking is  working correctly. [Marc] Just to annoy.

But we found that the box was generating  the sub-bits, I'm hooked up to them.   Do you want to hit one of the - hold on a second -  hit one of the tapes? [Ken] Okay. [Marc] Okay,   so it just sent that. [Ken] So, in  yellow, you can see the the bit pattern   it's sending. And then in green is the sub-bits.  So each yellow bit has been expanded into   five green sub-bits. So for instance, here you  can see the pattern for a one sub-bits, and then   the zero turns into a different pattern of  sub-bits, the complement. [Marc] You want to  

load another tape that's just ones or zeroes,  so we can see the beginning of the sub-bits?   There you go. We have the twos. Okay, so  here we have a bunch of twos, for example.   And I was going to move at the beginning  of the sequence, and it's different.   This whole beginning of the sequence  here is encoded completely differently. [Ken] So you can see that at the beginning,  the zero turns into a one-one-zero-one-one   sub-bit sequence, which is  entirely different from the   zero sequence you get here. So that  they can tell where the beginning of the   of the message is. [Marc] Okay. [Ken] And  if we zoom in, we can see the PSK signal. [Marc] Oh, no! Now we have our encoded  messages, then our bits divided into sub-bits,   but that’s still not what we need to transmit.  What is transmitted is actually PSK, or  

Phase Shift Keyed signals. I explained it in video  12, when we made our modulation infrastructure.   So let’s restart the elevator music for a  short review, because this is not simple. So we have our message bits. We break them down  into sub-bits. We then take a 2 kHz carrier,  

and use PSK to modulate the sub-bits onto it.  This gives the weird waveform where the phase   abruptly shifts by 180 degrees when there is  a bit transition. And to ease the decoding,   we add a phase synchronous reference signal at 1  kHz. Which gives that contorted modulation signal.   Our Updata box is supposed to do all that,  except it does not, because we are missing   part of these circuits. So I found a roundabout  way to recreate it with HP / Keysight equipment.   And I was quite surprised and relieved when I got the  exact same contorted signal on the scope.

So let me explain for a second how I do that. So,  we are missing parts, and so we don't have the PSK   generating stuff working quite yet. So I  pick up the green bit which is the sub-bit,   and that goes over to my Keysight  equipment. And that one has   a two kilohertz, on which we do the PSK, and a  one kilohertz, which is just a reference tone. And there is one more crutch to that box, it's  back there. We use some friendly 1960s equipment,   because this box is missing is its 4  kilohertz clock. [Ken] Here you can  

see the 4 kilohertz. [Marc] And it's not any 4  kilohertz, it's 4 kilohertz that's phase-locked   to the 2 kilohertz of the Keysight. Because you  have to phase-shift it at exactly the right time.   So those two boxes have to be synchronized. And  that's the box I just repaired, and I used the   PLL capability of that box. [Eric] Did you  do any other phase adjustments between that   reference 4 kHz and the 2 kHz? [Marc] Yes,  you need to. Actually, you can tell that   I'm not perfectly in phase here - I'm pretty  close actually! But yes, there is a button   that allows you to choose exactly. Not only  lock on the right harmonic, but then you choose  

which phase you have to be in. [Eric] I see, okay.  So normally when this is generating the signal,   that 4 kilohertz clock comes in, and that's  used to generate the 1 kHz and the 2 kHz,   everything else you need. But in this case, you  can't really do that. So you've got an external   instrument that's generating your 1 kilohertz and  your 2 kilohertz. And then to get the 4 kilohertz,   you run that into your little 1960s PLL. [Marc]  I need the help of some Nixie tube stuff! [Ken]   So there's supposed to be a 4 kilohertz oscillator  on this board, but somebody took it. [Marc] Right. [Ken] Do you want to put on the sound  so people can hear what the message   sounds like? [Marc] Yes, let's... So we are  actually transmitting! So then, since we had the  

Keysight equipment, and it's already transmitting an uplink - right now it's   transmitting just idle. So you can see  it here. And that sounds like a tone. And then, if you send something. [Ken] Okay! [fax-machine like sounds] [Marc] It just went 'meep'! Okay, do it again. [more fax-machine like sounds] Oh that's the three, okay. [higher pitched tone] [Marc] Beep! [Ken] That sounds a little different for  the different messages. [Marc] Right.

[Ken] We need to put a song on here somehow. [Marc] Let me try to send a sequence. [bursts of fax-machine like sounds] So I've made a very special tape of Apollo  commands. Let's see if we can... Ah,   it's changing as we speak! So there is really one  module not connected properly, is my guess.   So we have to re-seat all the modules.  'Clear'. Okay, put it on 'Start'.

Zero. 4601: it's a nasty message. It's Abort  A Light. So I'm sending Abort A. Abort B light. Crew Alert. And then finally, I want to cool things  off. And I send Reset Message 30,  

Reset Message 32. And those will reset  the ship propulsion and all the alarms   to zero. So now, after we've woken up the  astronauts, we say: oh, no, never mind! That will reset telecoms: that's a  Reset Message 70, Reset Message 72.

That will do relay 22 and relay   27 - see it changed right there - relay  27. And that's to turn ranging on. Test Message A, Test Message B. So, if I redo this one, and if the box doesn't  die on us. I'm just going to do the whole -   okay, we're at zero. I'm just  going to do the whole sequence.  

Then we're going to listen to it. Here we go. [mix of tones and fax-like bursts] Yeah, it's hard to hear any  difference, they all sound the same. Okay anyhow, that was our first transmission  of commands. Now we have to decode them. We actually did get a little bit  further, transmitting and recovering   sound and data at the same time, showing  that there is no crosstalk between the two.  

As usual, we’ll update you in  the next episode. See you then!

2022-03-26 12:45

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