I recently bought a lot of old teaching materials from the 80’s, among them set 9765 that consists of an interface cable for Commodore 64/128 and an instruction booklet. The cable connects LEGO’s electronic “Interface A” from set 9750 (and 1093) with the user port on the computer. The only software included are the examples in the book.

I have scanned the book and you can get it from here (76 pages, 24 MB).

If you want to make your own cable, you can see the necessary connections here:

3 -> 2 (+5VDC)
5 -> 1 (GND)
6 -> C (PB0)
8 -> D (PB1)
10 -> E (PB2)
12 -> F (PB3)
14 -> H (PB4)
16 -> J (PB5)
17 -> 12 (GND)
18 -> K (PB6)
20 -> L (PB7)

You need one “ribbon cable with 20 pin connector” and one “Card Edge Connector socket (3.96mm)”.

(Click the picture for high resolution)