Dual Keyboard switch
© 01-Oct-2000 13:07 Dr. Spiff

Here is an arrangement that allows you to use 2 keyboards from the same PC, at the same time. Using a simple "Y" connector won't do the job as the the computer interogates the keyboard at bootup. If the BIOS sees anything other what is expected, you will get the dreaded "F1 Keyboard Failure". There are really two parts to this adaptor. The double-pole double-throw switch (DPDT) hides the slave keyboard from the CPU during bootup and the 7400 (Quad 2-input NAND) helps to prevent data contention errors between the 2 keyboards.

I built my adaptor in a small box using a keyboard cable as the input and 2 bulkhead mounted keyboard receptacles that my two keyboards plug into. Inside the box, I used point-to-point wiring as there are only a few connections that need to be made. The drawing below is "reverse-engineered" off the box I built several years ago. You will notice that 3 of the 4 pins are wired straight through from the computer to both keyboards.

The 7400 gets its power from the +5vdc that the CPU sends to the keyboard encoder chip. To get the system running with 2 keyboards using this adaptor, follow these steps:

1. Set the DPDT switch to the "MASTER" position
2. Boot up the computer
3. Once you get up and running, Set the DPDT switch to the "BOTH" position

That is all there is to it.