I found a 40 pin connector on the side of one of my old electric typewriters. There is zero documentation of this plug online or in the owner's manual, and I want to figure out what exactly it can do. I have found several ground pins by doing a continuity test with a labeled ground elsewhere on the circuit board, but am unsure about what to do next. Specifically, could I safely use a Arduino or something similar to determine the voltage of the pins as I'm actively using the typewriter, or trace the circuit board to reverse engineer its outputs/inputs? If so, how? I have a suspicion that it is used for digital output/input of keystrokes.

The typewriter is a SR 3000 Electronic graduate (Model number 161.530).

Here is the top of the board top of the board and the bottom of the board bottom of the board and a close up of the connector in question, which is seen on the bottom right of the first picture. 40 pin plulg

Any help would be really appreciated.

  • 2
    Connector placement/access to the inside/outside of the case and any case labeling would provide some hints.
    – Brian
    Jan 14 at 1:53
  • What's the retrocomputing connection here?
    – dave
    Jan 14 at 2:40
  • @another-dave: If this device was intended for use as a computer peripheral, I would think it would almost certainly be old enough to qualify as "retrocomputing".
    – supercat
    Jan 14 at 3:23
  • 5
    Not so hard to find facts: Mitsubishi M5L8049 is a microcontroller with 2k on board ROM and 2 8 bit I/O ports (plus 8 bit databus) and 5v VCC. Reverse engineerign of the PCB should be quite trivial, since it seems to be one layer. It ssems to have ben made by NAkajima Typewriter in Japan, so the same model might very well exist with other branding.
    – UncleBod
    Jan 14 at 7:46
  • 3
    This forum discussed using the SR2000 with PC and C128. It might be close enough? Jan 14 at 9:27

1 Answer 1


With the continuity checker, a pencil, and graph paper, you can usually reverse-engineer a single-layer circuit board like this without even powering it on.

First you reverse-engineer the PCB into a netlist by figuring out which chip and which pin of that chip each pin of the connector is wired to. You can use this netlist to build a schematic, if that helps.

Then you look up the function of each of those pins in the chips’ datasheets. That will often give you enough of an idea of what that connector pin is meant for.

If the chips have “mystery” designations, you will have to work at this recursively, inferring their function by what they’re connected to, and so on. Eventually you will reach a component (like a keyboard, CRT neck, floppy drive cable, LED, serial port, etc.) that will be identifiable. Other times you get lucky, and can find a mapping of internal part numbers to their external equivalents.

It can be fun to publish your work, even if you don’t 100% complete it. It will help others who follow in your footsteps! I started this with a HP4951b a few years ago.

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