It was interesting to see in page 10 of the 1:1a issue of SyncWare News, a joystick circuit that is a variation of a circuit which I have been using since I customized my Sinclair ZX81 in the summer of 1982. When I moved the ZX PCB in a large keyboard case, together with a HiTek 53-key professional S keyboard, I brought the 13 KBD and DIODE lines out on a 16-pin connector, together with +5V and ground.
I have since been using encapsulated circuits as per the schematic below. An Atari joystick plugs into the circuit, which in turn plugs into the 16pin connector. The circuit uses a Quad Analog Switch, such as the 4016 or 4066. If you’ve experimented with this, you know that the KBD/Diode matrix does NOT require a low-resistance connection for a character to be formed. As a matter of fact, the “contact” resistance can be as high as 1500 ohms for reliable operation. The “ON”-resistance of the logic switches is anywhere from 100 ohms to several hundred ohms, depending on the “enable” voltage. The circuit has the advantage over the transistor. version that no experimentation with resistor values is required, and it is cheaper; a quad switch typically sells for 99 cents, against four transistors for at least two dollars… [Of course, that depends on how and where you buy your parts ed.]
So far, I have not had a need for more than four control lines per circuit, but you could of course use more devices to give additional control lines.
EDITOR’S NOTE ~ An additional advantage to this type of circuit is that it has somewhat better noise immunity than the transistor circuit. If you have to remotely control the keyboard from a great distance, then the recommended technique is to use optocouplers to do the switching, and the lines that earry the DC to the LED portion of the coupler can be practically as long as you like. The coupler effectively isolates out any ground-loop-induced noise, etc.
