ADM-3A question

Charles charlesmorris800 at
Fri Aug 16 15:43:47 CDT 2019

Today the replacement 'LS193 arrived, so I put it in the 
previously-installed socket and the screen is now 24x80 again :)
I'd been testing with the dip switch in half-duplex mode... For final test, 
I put it in FDX, connected to my HP protocol analyzer, and what do you know, 
no serial data out.
The 1488 RS-232 driver was blown (TTL-level data going to it, but the RS-232 
line was stuck in spacing). Not an uncommon failure with static discharge 
and incorrect cable hookups...
I destructively removed it, again installed a good dip socket, now waiting 
for THAT chip (a variety of other line drivers in my drawer, but not 

Meanwhile I noted another slide switch S8 ("GT/LK") near the DB-25 
connectors. It is not referenced anywhere in the documentation, nor in the 
The wiper of the switch also goes through a hex inverter to a 74LS32 chip, 
ALSO not in the schematic or circuit description. This signal originates at 
the flip-flop that generates KBLOCK\.
Finally, input pin 10 of the removed 1488 is supposed to be tied to pin 9, 
with the RTS output on pin 8. But pin 10 goes In fact, the PCB artwork at 
the end of the tech manual shows no connections except +5 and ground to that 
chip (position C2 I think), and it doesn't show the slide switch either.

This is likely something for the auto-tester that LSI used to check these 
boards on the production line, although I don't know if the extra circuitry 
was added or removed during production.
Anyone have internal documents on this? I'm just curious since it only 
appears to affect the keyboard lock functions which I'm not using anyway.

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