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teenix · (This post was last modified: 10-08-2019 06:42 AM by teenix.)

(10-08-2019 06:03 AM)albertofenini Wrote: Hi Tony
that's a good start, can you please help us in telling what should we measure ?
I mean between which points should we put the probes and what to expect when a key is pressed ?
We have never done it and it's a good thing to learn !
thanks in advance !

I'm pretty sure each of the 8 row outputs go logic low in a sequential manner. These pins are marked on the hp65 schematic. Then the 5 columns are tested and if any are logic low, one or more keys are pressed. This is a typical multiplexing arrangement which reduces the pin count for chips and connections..

Place an oscilloscope probe on row 1 (pin 18 of keyboard connector), for monitoring the A B C D and E keys. When you see a low signal, the CTC is now checking these to see if any of them are pressed.

If any or all of these keys are pressed, you should see a matching logic low signal appear on the corresponding column input.

For example, when row 1 is low and the A key is pressed, column 1 should also go low.

cheers

Tony