14 Dec
2015
14 Dec
'15
7:59 a.m.
Some more random thoughts....
Of the above 10 pins, 11, 13, 22 & 23 are high at power-up (printer
NOT connected to any interface). The only pin with known function is
19, which is 'paper out'; if I toggle the paper out switch I can see
it going high and low.
Well, 19 could be a general printer-ready pin in that if the paper runs
out it would say the printer is not ready for another character but
it might well also be put to the not-ready state when the printer
was printing the current character. Seen that before.
Also, having now looked at the photos (sorry I was rushed before) I
am pretty sure that Allen Bradley package is a pull-up resistor
array. Half the pins seem to go to the +5V trace. The others go
to signals
Remember that a pulled-up logic input will test as a high level. So some
or all of the 'high' pins might be inputs.
I notice the 3 chips with the Pn lables. I think I can make out a Harris
logo on one of them. I would guess these are programmed PROMs
to convert between ASCII (I hope) and the solenoid codes for the
Selectric.
The next thing of interest to me is the pair of 7475 latches at the bottom
of the board. 4 bits each. Maybe hold the 8 bit input character. I would
trace where the D's and Q's of those go first.
Perhaps you load the character a nybble at a time???
This board does not look that complicated and all the ICs have known
numbers on them (mostly TTL logic). If it were mine I'd trace out the schematic.
-tony