Note: the same cable also applies to the Mac 128 and Mac 512.
If your Mac Plus like mine was missing the cable that connects it to it’s keyboard and you wish to make a replacement cable yourself you will need to get hold of 2 RJ10 connectors (4P4C), don’t get the more common RJ11 as though they are 4 wire they are slightly wider. You will need a crimper tool (sometimes a RJ11 crimper will be fine) and some flat telephone cable. Some of you may have noticed that the cable is the same as the telephone cable that connects handsets to the phone, unfortunately though this type of cable and pretty much any type of pre manufactured cable you find is wired wrong for the Mac Plus.
Under no circumstances should you use this cable as you will damage your keyboard and/or your Mac!
To check a cable for suitability hold the 2 connectors so they are facing the same way, and are the same way up and that you can see the colour of the wires, a telephone cable will have the colours on opposite sides.
A Mac Plus keyboard cable will have the colours on the same pins on both connectors, for example:
Pinouts of the Mac Plus keyboard connector, looking into the mac plus keyboard connector:
1 – GND
2 – KBD1 (clock)
3 – KBD2 (keyboard data)
4 – +5V DC
So as you can see connecting up with the wrong cable is going to apply 5 volts in reverse polarity to the circuitry in the keyboard!