30 Sep
2015
30 Sep
'15
10:46 p.m.
Hey guys,
I'm fairly certain this is a serial board - mostly because the PO told me
so, and when I received it, it had a 25-pin male connector on a three-wire
cable carelessly soldered to the pads behind one of the cable headers on
the top edge. But the cable was removed as a matter of course when I was
prepping the machine for a rebuild. I mistakenly assumed that the docs
would be trivial to locate - so no need to record the original wiring
connections.
.
On Thu, Oct 1, 2015 at 12:26 AM, tony duell <ard at p850ug1.demon.co.uk> wrote:
I couldn't spot how they did level shifting
to RS232 levels. The SI
signal appears to go to a 7400 next to one of the Uarts.
There are a couple of 8 pin ICs near the 10 pin headers, they look to
be 1458 dual op-amps (I can't exactly make out the numbers...).
Op-amps were often used as RS232 drivers. Give them +/-12V supplies,
feed a TTL signal into one of the inputs, connect the other input to a
voltage between the TTL levels (potential divider from the 5V line) and
apply no negative feedback. The Op-amp output will swing between the
supply rails.
-tony