Electronic Systems TRS-80 Serial I/O Board?

jim stephens jwsmail at jwsss.com
Tue Jun 13 00:23:18 CDT 2017

On 6/12/2017 5:08 PM, Win Heagy via cctalk wrote:
> Does anyone recognize this serial board for the TRS-80...Model 1 I think.
> It came with a bunch of model 1 stuff years ago.  I haven't been able to
> locate much online.  Some manual type material would be great.  Nothing on
> the back except "Copyright 1978 Electronic Systems".
> http://imgur.com/a/m5YiV
> Thanks,
> Win
Things odd about the board.  No 1488 / 1489 or any other transmit / 
receiver chips.  No clock circuit.  Was there an appropriate frequency 
crystal or clock on the interface to divide to something for the 1602 
uart clock?

I see transistors randomly across the bottom and "EIA" in the etch, 
maybe they got cheap on the RS232 / TTL.  The CD4069 cmos chips are the 
only things other than the 10 turn pots I see that are other than TTL 
and the UART.

1602 Datasheet:


1981 data book, PDF page 20

Pin 20 RI Receive
Pin 25 TRD Transmit
Pin 40 transmitter clock
Pin 17 receiver clock

Anyone having a reference for the connector at top might help especially 
with the clock.  Since it is a UART, look
for where the TRE and Receive ready bits are and you probably can run 
the thing.  (and of course figure out where
the data register is WRT to the register with those bits).  Full modem 
control doesn't look likely with this thing,
but some flow control can be accomplished with another bit or two.

where they go might be interesting.


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