The keyboard is attached via a DB37 (no documentation on what signal/power is on there)
The keyboard processor board has two 5v regulators on it and its logic chips seem to have +5 OK - I was hoping to find just a shorted tantalum killing a power rail, but no such easy problem! (and the damn thing is conformal coated so a pain to make measurements.)
It looks like the keyboard itself and the fluorescent character display are standard commercial parts - they are connected to the processor board via ribbon cables
Access is via the bottom of the keyboard so it is pretty awkward - I may make an extender cable so I can more easily probe things.
First I will look at the Z80 to see if it active (I think it must be because during power-on the character display gets initialized and then it says "PLEASE WAIT". Then the printer finishes its power-on sequence and prints "READY", but the BIT status code says "keyboard not installed",
Then I will look at the TX and RX lines on the keyboard USART that talk to a USART in the control unit.
There must be some attempted handshake during power-up BIT because the BIT status at that point says "keyboard not installed". And thereafter the internal software thinks this is an RO with no keyboard.....
But I got no flow diagram or much about how the controller, keyboard, and printer microprocessors actually interact, other than that they use serial links.
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Keyboard unit A3
applies serial data directly to backplane interface A2A1A3A1, backplane A2A1A1, I/O connector assembly
A2A1A2 and I/O connector assembly A1A4 to the receiver section of a USART on port card A1A2A1 of electronic
control unit A1. The electronic control unit A1 USART converts the data back to parallel form for processing
by the IOC microprocessor and storage in the dual port RAM of DSC A1A2A3. Based upon receipt of
KYBD ONLINE data in the dual port RAM, the storage control functional group ( paragraph 3-4) develops
appropriate control signals. Control signals are sent back to keyboard unit A3 by reversing the path followed by
the KYBD ONLINE data except for the substitution of USART transmitter section for receiver sections and
receiver sections for transmitter sections. Character keys typed by the operator, thereafter, follow the same path
as the KYBD ONLINE data to be stored in the DSC dual port RAM. The control microprocessor on the DSC
(figure FO-5-3) transfers data in the DSC dual port RAM into the print and communications lists of the buffer
memory. From that point keyboard originated characters are transferred to the print and transmit buffers in the
dual port RAM for routing to the message print and message transmit function circuits.
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