I have a Technic Interface A with the cables but lack the printed circuit board (PCB) for PC that I need to use it for automating my models. The card when it was for sale had markings of Lego 9771 and I assume this to be the part number.

Ideally I would like an original card but is there a way of getting a 3rd party card? Or another idea might be if someone does manage to make such an interface please consider building it to run multiple Interface A and/or B modules simultaneously.

I think that I may have parts that run on almost if not all of the powered interfaces that Lego has been kind enough to release. I would be quite grateful indeed if I were able to use them together in my sizeable Lego projects.

  • Hi Mark and welcome to Bricks.SE! I for one have a hard time picturing your situation as I haven't heard of "Technic Interface A", so could you please upload a photo of your part? The 9771 number does not match any known sets and none of the matching parts are related to Technic.
    – zovits
    Aug 12, 2020 at 8:12
  • 4
    Does this help? Technic Interface A on lgauge.com
    – Uli
    Aug 12, 2020 at 8:30
  • @Uli Thanks, I have learned something new today :)
    – zovits
    Aug 13, 2020 at 9:26

1 Answer 1


Uli posted a link in a comment to https://lgauge.com/technic/LEGOInterfaceA/9750(1093).htm which has a link to http://isodomos.com/Lego-Sets/1093.html , that link is dead but archive.org has a copy http://web.archive.org/web/20081121054400/http://isodomos.com/Lego-Sets/1093.html both sites have the pinout, but the latter site also confirms the logic levels as 5V TTL.

I will repeat the pinout here, in case those sites disappear.

  • Pins 1 and 3: 5V

  • Other odd numbered pins: Ground

  • Even numbered pins 6-16: Outputs 0-5

  • Pins 18 and 20: Inputs 6 and 7

Unlike the interface B which is a somewhat-intelligent device that communicates over RS-232 the interface A seems to just use straight digital IO lines. For bi-directional control of motors the outputs can be used in pairs.

The pinout seems to be an exact match for the user port on a BBC micro. I suspect this is not a coincidence.

It would not be difficult to hook this up to something more modern, a 5V Arduino could probably hook up directly. For a Raspberry Pi you would want something to shift the levels, I'd probably use HCT logic running off 5V for the signals from the Pi to the interface A (this may not be strictly needed) and LVC logic running off 3.3V for the signals from the interface A to the Pi.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.