Unitron 2000 keyboard

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Last seen: 1 year 10 months ago
Joined: Nov 24 2013 - 13:23
Posts: 226
Unitron 2000 keyboard

I have this exact keyboard here.

Some years ago, while I was trying to trouble shoot problems with an A2S1 not typing, I tried using it there and something must have blown up, because after that it wouldn't work with anything.

I've done some troubleshooting with a logic probe and am not finding a clock signal on the 74273 pin 11, which I'm thinking should be there. I had originally thought I'd blown the EPROM, but when I probe the output legs there is activity when keys are pressed.

I've also accidentally shorted pins on the 273 or the 74123 below and had it burp out stuff at the prompt.

A schematic for this thing would be ideal, but I've not yet found any. Since we know the motherboard is basically a clone of the Apple II board, I thought maybe one of you would know -- pin 11 on that 273 seems to connect to pin 2 on the keyboard header, which according to the II schematic is STROBE -- would it typically be the keyboard generating the clock/strobe there? Or would that come up from the logic board?

Last seen: 1 month 3 weeks ago
Joined: Aug 4 2018 - 21:59
Posts: 139
Yes the strobe signal is

Yes the strobe signal is generated by the keyboard to signal that a key has been pressed and the data is good

I don't have a schematic but can guide you through how it works based on what the chips are.

Starting with the NE556 one half is used to generate the clock that drives everything and the other half generates the key repeat rate clock.
The main clock of the NE556 is fed into the 74LS93 (decade counter), so you should see that counting even when no key is pressed.
The 74LS93 outputs are fed into some of the EPROM address lines (for decoding scan line data to ascii) and also the 74LS42 (1 of 10 decoder)
The 74LS42 takes the BCD input and is used to drive one scan line at a time. You should see it's outputs activating in sequence.
The keys are connected on a grid of rows and columns, the 74LS42 will drive either the rows or columns and the other is wired to the EPROM Address lines to complete the decoding.
The EPROM is a seven bit map of the rows and columns, the final bit is used to start the strobe sequence.
Address lines of the EPROM are fed by the rows and columns of the key matrix and the data lines are 7bit ascii and strobe trigger.
The strobe trigger bit is used to latch the data into the 74LS273 and trigger one half of the 74LS123.
The 74LS123 is used to set the strobe pulse length and the other half is used to set the key repeat delay time.

I can't be more specific and give pin numbers because I don't have a schematic or a keyboard. The theory of operation is based entirely on what the chips are.

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