C128D Keyboard for C-64

Project: C128D keyboard as substitute for original C64 keyboard.
Target : C64
Time   : 90-180 min.
Cost   : ~10 US$
Use    : - C-128 keyboard is more ergonomic
         - up to 10 meters cable length without killing the CIA

Summary

C-128 keyboard parallel to original C-64 keyboard with a keyboard booster circuit.

Details

Simply replacing the original keyboard by a C-128 D one would be dangerous for the CIA. To save lines, the CIA is wired as a matrix, so that the controlling is dynamic. The CIA has to supply the necessary currents for loading the cable capacities. The longer the cable, the higher the capacity. The maximum output current varies with the manufacturer; overloading the CIA with a capacity exceeding the specified output current will destroy it.

Ingredients

1 C-128 D keyboard
1 25-pin subminiature connector (female)
1 20-pin 1-row direct pcb connector (female)
1 20-pin 1-row direct pcb connector (male)
1 20-core flat cable
1 74HC541
1 20-pin high precision IC socket
8 diodes 1N914 or 1N4148
8-resistor-array (9-pin version) or (better not) 8 single resistors, 10k ohm

Instructions

Pull off all cables, then open case.
Build the circuit shown below:

(Click here for ASCII version)
Now solder the other end of the flat cable to the 25-pin connector according to the following scheme:

(Click here for ASCII version)
Close the case again.

Possible failures

Soldered wires to wrong pins on one or both of the connectors. The figure above depicts the plugs from the SOLDER side and the jacks from the PLUG side, respectively.
Cable is too long -> shorten cable (1 m should work fine).

Updated: September 30th, 1997
Created: Early 1997
Status : Verified