Expanding The Keyboard Of The Nascom
Using Normal Keyswitches

by J. M. H. Hill

An article was published in issue number 2 of Micropower on the expansion of the Nascom 1 keyboard. Now although the keyboard uses special Licon keys, it is possible to expand it using ordinary push-to-make keyswitches. I have been using such an expanded keyboard for about a year without any problems, and there seems to be no reason why the principle should not be extended further if desired to cover a separate numerical keypad.

The only modifications to the existing keyboard conductors is the cutting of the tracks running between the open collector outputs of IC5 and the original key matrix, to allow the insertion of the isolating diodes D3 – D8. The presence of these siodes does not affect the operation of the original keys. Their purpose is to prevent the voltages of the inactive drive lines from being affected via the Licon key windings by others which are active. A more ambitious expansion would also involve the other two outputs from IC5 on pins 7 and 9, which would also need to be fitted with isolating diodes.

To duplicate any existing key, as in the case of the SHIFT key shown in the circuit, it is necessary to connect the right drive line output from IC5 via a keyswitch and diode (D9) to the input of the appropriate RS flip-flop. The connections are shown for the SHIFT key, but others can be worked out by examining the Nascom 1 keyboard circuit diagram. A further locking keyswitch could be connected in parallel with the SHIFT key to give a shift lock if required. Such a shift lock could be added to the standard Nascom 2 keyboard to provide the facility requested by Mr. R. C. Taylor in issue 2.

Duplication of the standard keys will work with any monitor, but the remaining keys shown in the diagram are for use with Nas-Sys 1 or 3. They give most of the facilities of the Nascom 2 keyboard, inluding single key cursor movement. The Graphics key will of course only work if a suitable graphics unit has been fitted.

The standard Nascom 1 keyboard only uses the lowest six bits of port 0. To handle the extra keys a sense line using bit 6 is needed, plus an extra flip-flop. Fortunately, there are two unused gates in IC3 which can be used for this purpose, as shown in the diagram. An extra wire will be needed to connect the output from the new flip-flop to pin 7 of the keyboard socket on the main board.