4 ms·

Reverse engineering circuitry in a Spacelab computer from 1980

123 points by elpocko 4mo ago

kens 4mo ago
Author here if anyone has questions...
- monocasa 4mo ago
  When you say that the 125 added memory management, what does that mean a little more specifically? My guess is either PDP-11/z280 style paging without page tables (the 16 bit address space makes just having enough IO registers to cover the address space tractable) or some simple segmentation hardware, but it'd be neat if there was another hardware object capability system I didn't know about.
  - kens 4mo ago
    The Mitra 125 had memory segmentation. It seems similar to the 8086, with descriptors that specified base address and length for a segment. Accessing memory outside a segment caused a trap.
    - atq2119 4mo ago
      That sounds more like the 80286/80386.
      - kens 4mo ago
        You're right; I should have said x86-style.
- skissane 4mo ago
  Do we know anything about the software that ran on it? What language it was developed on, what host platform was used for development, etc? Does any of the code survive?
  - kens 4mo ago
    Much of the software was in a language called HAL/S (High-order Assembly Language/Shuttle), using an IBM 360/370 for compilation. Other software was written in Fortran. See https://www.cs.toronto.edu/XPL/hal.html https://www.cs.toronto.edu/XPL/hal.html and https://repository.arizona.edu/bitstream/handle/10150/613923/ITC_1979_79-09-4.pdf https://repository.arizona.edu/bitstream/handle/10150/613923...
    - skissane 4mo ago
      Interesting... I know about HAL/S being used for the AP-101S Shuttle software, inter alia. I didn't realise it was used for the CII Mitra 125 as well. I would have assumed the Europeans would have used something else.
- tasty_freeze 4mo ago
  What frequency did it it cycle at? Any idea how many instructions per second it could process?
  - kens 4mo ago
    I don't know what the clock speed was, but it performed about 525,000 addition instructions per second. (1.9µs addition time)
GlibMonkeyDeath 4mo ago
The PCB construction is curious (which you say is multi-layer) - why use a grid of 0.1" holes? Is that so it could be easily jumpered? Can you tell if the traces run through the holes or between them? I don't have the patience to reverse-engineer these types of boards, but I do find them really interesting to think about. CAD was just getting started (I just looked up that Gerber format was released in 1980) so I wonder if the masks were hand-drawn.
- deleted 4mo ago
  [deleted]
- kens 4mo ago
  The grid is 0.1" holes because that's the spacing of most components; it's a bit like perfboard. If you're asking why they used a grid instead of a normal-style PCB, I'm not sure. It probably makes manufacturing the boards much easier since you can drill the holes with an assembly line rather than one at a time. The traces go between the holes; the traces are very narrow, so two traces can fit between a pair of holes. That's probably the tradeoff, that your traces need to be very precise and you probably need more layers because of all the holes in the way. The layout was probably done with CAD; PCBs with CAD go way back. IBM was doing circuit boards with CAD back in the early 1960s, using a flying spot of light to draw out the PCBs on photoresist.
  - GlibMonkeyDeath 4mo ago
    >The traces go between the holes That's what I was curious about - otherwise there would be effectively be through-vias along the traces (so traces could be probed between devices.)
    - kens 4mo ago
      You can see how the traces go between the holes if you zoom in on this photo: https://static.righto.com/images/cimsa/board-back.jpg https://static.righto.com/images/cimsa/board-back.jpg
pickleballcourt 4mo ago
Did not expect such an insanely detailed blog, crazy how even the computers for Spacelabs had such history.
CobaltFire 4mo ago
I have zero experience with anything in the space program, but this all looks REMARKABLY like the F/A-18 Avionics I worked on for years. The board connecters look identical to what most of those boxes used, as well as the test benches (I'm specifically familiar with the AN/USM-484). The circuit board designs, etc. all have a ton of similarities as well. I imagine it was a function of the design constraints in that time period, and similar program needs. There are very few records of any of this out there, similar to the Spacelab equipment I imagine. The -484 ran a Harris H100 minicomputer with an HP terminal and a GPIB bus. I've linked one of the few photos that you can find, but it's from an extended version of the test bench and has the minicomputer rack off to the right cut off. Sorry for the off topic, but it's always cool to see stuff like what I spent years working on come across this site! https://api.army.mil/e2/c/-images/2007/02/12/2612/army.mil-2007-02-13-114036.jpg https://api.army.mil/e2/c/-images/2007/02/12/2612/army.mil-2...

123 points by elpocko 4mo ago

kens 4mo ago

Author here if anyone has questions...

monocasa 4mo ago
When you say that the 125 added memory management, what does that mean a little more specifically? My guess is either PDP-11/z280 style paging without page tables (the 16 bit address space makes just having enough IO registers to cover the address space tractable) or some simple segmentation hardware, but it'd be neat if there was another hardware object capability system I didn't know about.
- kens 4mo ago
  The Mitra 125 had memory segmentation. It seems similar to the 8086, with descriptors that specified base address and length for a segment. Accessing memory outside a segment caused a trap.
  - atq2119 4mo ago
    That sounds more like the 80286/80386.
    - kens 4mo ago
      You're right; I should have said x86-style.
skissane 4mo ago
Do we know anything about the software that ran on it? What language it was developed on, what host platform was used for development, etc? Does any of the code survive?
- kens 4mo ago
  Much of the software was in a language called HAL/S (High-order Assembly Language/Shuttle), using an IBM 360/370 for compilation. Other software was written in Fortran. See https://www.cs.toronto.edu/XPL/hal.html https://www.cs.toronto.edu/XPL/hal.html and https://repository.arizona.edu/bitstream/handle/10150/613923/ITC_1979_79-09-4.pdf https://repository.arizona.edu/bitstream/handle/10150/613923...
  - skissane 4mo ago
    Interesting... I know about HAL/S being used for the AP-101S Shuttle software, inter alia. I didn't realise it was used for the CII Mitra 125 as well. I would have assumed the Europeans would have used something else.
tasty_freeze 4mo ago
What frequency did it it cycle at? Any idea how many instructions per second it could process?
- kens 4mo ago
  I don't know what the clock speed was, but it performed about 525,000 addition instructions per second. (1.9µs addition time)

GlibMonkeyDeath 4mo ago

The PCB construction is curious (which you say is multi-layer) - why use a grid of 0.1" holes? Is that so it could be easily jumpered? Can you tell if the traces run through the holes or between them? I don't have the patience to reverse-engineer these types of boards, but I do find them really interesting to think about. CAD was just getting started (I just looked up that Gerber format was released in 1980) so I wonder if the masks were hand-drawn.

deleted 4mo ago
[deleted]
kens 4mo ago
The grid is 0.1" holes because that's the spacing of most components; it's a bit like perfboard. If you're asking why they used a grid instead of a normal-style PCB, I'm not sure. It probably makes manufacturing the boards much easier since you can drill the holes with an assembly line rather than one at a time. The traces go between the holes; the traces are very narrow, so two traces can fit between a pair of holes. That's probably the tradeoff, that your traces need to be very precise and you probably need more layers because of all the holes in the way. The layout was probably done with CAD; PCBs with CAD go way back. IBM was doing circuit boards with CAD back in the early 1960s, using a flying spot of light to draw out the PCBs on photoresist.
- GlibMonkeyDeath 4mo ago
  >The traces go between the holes That's what I was curious about - otherwise there would be effectively be through-vias along the traces (so traces could be probed between devices.)
  - kens 4mo ago
    You can see how the traces go between the holes if you zoom in on this photo: https://static.righto.com/images/cimsa/board-back.jpg https://static.righto.com/images/cimsa/board-back.jpg

pickleballcourt 4mo ago

Did not expect such an insanely detailed blog, crazy how even the computers for Spacelabs had such history.

CobaltFire 4mo ago

I have zero experience with anything in the space program, but this all looks REMARKABLY like the F/A-18 Avionics I worked on for years. The board connecters look identical to what most of those boxes used, as well as the test benches (I'm specifically familiar with the AN/USM-484). The circuit board designs, etc. all have a ton of similarities as well. I imagine it was a function of the design constraints in that time period, and similar program needs. There are very few records of any of this out there, similar to the Spacelab equipment I imagine. The -484 ran a Harris H100 minicomputer with an HP terminal and a GPIB bus. I've linked one of the few photos that you can find, but it's from an extended version of the test bench and has the minicomputer rack off to the right cut off. Sorry for the off topic, but it's always cool to see stuff like what I spent years working on come across this site! https://api.army.mil/e2/c/-images/2007/02/12/2612/army.mil-2007-02-13-114036.jpg https://api.army.mil/e2/c/-images/2007/02/12/2612/army.mil-2...