Keyboard troubleshooting

[mykrowyre]

I purchased an Amber monochrome dumb terminal, but it came with a mismatched keyboard. Sold as not working.

I found that the pinout of the keyboard was different than the terminal pinout, so it fried the 7404N buffer chip by sending 12V to the data line.

Upon testing the keyboard with bench supply, I found the clock signal was being sent, but data was not as it that buffer pin was now shorted to ground.

I replaced the buffer chip, and the keyboard is working. But now I have a strange oddity.

The clock signal is now inverted. Instead of the clock pulses being negative (5V to ground), they are now positive.

The data pulses are negative as expected, so that is strange. All of the IBM keyboard protocol examples I can find show negative pulses on clock.

Unfortunately I have no information on the keyboard or the terminal, so I dont know which pins are clock and data on the terminal.

The terminal is a Liberty Aspect 4350. The keyboard only says "ASCII KEYBOARD TAIWAN".

Any input appreciated.


Thanks

 

[modem7]

The clock signal is now inverted. Instead of the clock pulses being negative (5V to ground), they are now positive.

You are using the word, "now". That implies that previously, you observed that the clock pulses were negative-going. Is that the case?

The data pulses are negative as expected, so that is strange. All of the IBM keyboard protocol examples I can find show negative pulses on clock.

There is no international standard.

 

[mykrowyre]

LOL oops I replaced the DM7407 (tri state buffer) with a SN7404 (hex inverter). DUH! Pinouts are the same, just one is inverting.

 

[mykrowyre]

You are using the word, "now". That implies that previously, you observed that the clock pulses were negative-going. Is that the case?


There is no international standard.

Yes, after posting I realized my error. Thank you!

 

[Chuck(G)]

Er, you may want to revisit your "fix". PC keyboard protocol (both XT and AT, as well as PS/2) call for open-collector driving on the signal lines, because either the host or the keyboard needs to be able to a line to ground. You should be using a 7406 or 7416 for that driver.

 

[modem7]

LOL oops I replaced the DM7407 (tri state buffer) with ...

I just looked up the 7407. Open-collector outputs.

 

[mykrowyre]

I just looked up the 7407. Open-collector outputs.

Yeah I confused myself flipping thru a pile of datasheets apparently.