At my work we have a bunch of old computer and logic stuff. I figured it would be best to ask about it here, since the original owners have long since retired. Any ideas what this thing is and what it was for?

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  • Are those LEDs or holders for incandescent bulbs? That might help set some date limits.
    – dave
    May 2, 2022 at 17:13
  • 1
    A photo of the back or innards may well provide more scope for analysis.
    – paxdiablo
    May 2, 2022 at 20:57
  • At first I thought it could have been the front panel from a PDP/8. Not the original one of course, but maybe a clone. Because it's twelve bits, and the MSB is numbered 0, and the LSB is numbered 11 (this is back-to-front from the modern convention). But this hypothesis does not explain the four-bit I/O connectors (would've been 6 bits on the PDP/8) nor the three power connectors, whatever that means. May 3, 2022 at 7:37
  • @another-dave They look like bulbs to me. I don't recall LEDs being common in the days when those types of toggle switches were in use.
    – Barmar
    May 3, 2022 at 15:41

3 Answers 3


Caveat: ATM I can not identify the machine this was taken from. I can only describe what can be said from parts visible. I will not go into further speculation.

Seems to be part of a front panel (i.e. blinkenlights) to some machine. Hard to say anything more detailed.

  • It's most likely a 12 bit system, as switches and lamps are organized as four groups of 3 each, which is typical for octal handling.

  • It can't be said if this is from a 'real' Computer, some IO processor or 'just' some controller, without further information.

  • Still, the 'POWER CONNECTORS' and 'I/O CONNECTORS' lamps seem to indicate that it's from some communication and/or I/O orientated system.

    • The 'POWER CONNECTORS' seem to be static assigned to (maybe) show activity on either line

    • The 'I/O CONNECTORS' (may) show activity on either line/device

  • The switches are most likely to set an address, which can be a memory address, some I/O register or channel address.

  • It is only intended for reading/checking data, as there is only a single set of switches and no visible controls to latch address and/or data (*1)

  • Operation is most likely (*1) done by

    • setting an address with the switches
    • switch position gets reflected on the lower row of lamps
    • any data at that address will be shown on the top row
  • Together ('I/O CONNECTORS' and 'data' lamps in single row) this might provide a nice display of actual activity. Quite useful to check if a certain functionality is operating the way it should (*2).

  • The 'LAMP CHECK' button does exactly that, ignite all lamps to see check for defects - quite common at a time when lamps weren't LED like today.

*1 - Unless there are not additional controls outside the picture, in another, missing panel.

*2 - Imagine the address being a status word of a serial interface, thus showing activity on either line, or conditions why there is no activity. Looking why a certain (modem) line is not operational (or why the remote user claims so) was one of the most common tasks.

  • 1
    Oh man, I'm left hanging by your (*1)!!!
    – Jon Custer
    May 2, 2022 at 16:52
  • @JonCuster I am sooo terrible sorry .... NOT :)))
    – Raffzahn
    May 2, 2022 at 16:54
  • 1
    Well, your footnotes are some of the most interesting parts of your answers!
    – Jon Custer
    May 2, 2022 at 16:55
  • Thanks! This device is totally separate from anything else, so no, there is nothing else around. However, that doesn't mean it didn't once belong to something bigger. May 2, 2022 at 16:58
  • @JonCuster Sorry to disappoint you, this time it's only to save ass, in case there's more visible outside the picture shown :))
    – Raffzahn
    May 2, 2022 at 17:00

A "switch register" or "sense switches" provides a way for the CPU to read a binary input without explicitly using the I/O bus. Due to the name "switch register," I would expect these switches to be mapped directly to a register accessible to the CPU. For a made-up example, if the CPU had 8 registers, one of them (let's say, register 0) would be mapped to the front-panel switches. Writes to that register would do nothing; reads from it would reflect the position of those switches.

A switch register was especially useful for interacting with bootstrap programs, for selecting the input device or other options, or for controlling diagnostic programs, but of course any program capable of reading that register could make use of them.

  • 4
    Early in my career, we called compiler and linker command line options "switches." I learned where that name came from when I did some work in Fortran II on an old IBM 1130 system: Compiler options were selected by flipping physical switches on the machine's front panel. May 2, 2022 at 19:50

I'm not familiar with this particular model but this type of "front panel" was found in many early mainframe, mini, and micro computer systems. While this example is among the simpler ones, and may be from a micro computer system, these all worked similarly.

They were used to program the system word by word and were often used to input a "boot loader" that would load the operating system from some other device like a paper tape reader or a disk drive.

These work by setting the memory address on the switches and then seeing the memory content at that address and then depositing a value set by the switches into that memory location. Often they had an "auto-increment" that would automatically increment the address to the next some so that you didn't have to manually enter the next address.

Overall it was quite tedious using these which is why they have pretty much disappeared except in some retro models out there.

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