Sorry to reply to my own post here, I had a bit more time to screw with
this this evening. Turns out it was a programming error (actually a
program entry error :)) that was (apparently) causing the 40mA current
instead of 20mA -- the little "echo" program I coded up wasn't properly
waiting for data to come in before echoing it back, so the 8/L was
constantly sending 377 back across the loop, which I'm assuming accounts
for the difference?
I realyl can;t see how the PDP8 sending anything (using its transmitter
and the receiver of the RS23 2adpater) can affect what happens in the
other loop. THe 2 sides should be totally independant.
With this corrected, the loop is registering ~23mA idle, dropping to
19-20mA when the PC attempts to send data. Can anyone comment on
whether this sounds reasonable? Anyone have any advice? (Getting
desperate here :)).
23mA is fine. But if 20mA is the 'off' state, tht is not fine. It could
be right for the mean current while data is being snet,
I have looked at the docuemnt you pointed out in another message. I
notice that this RS23 2interface powers itself from the RS232 handshake
lines. I am not overly happy with that, but... I owuld just check that
the right voltages do exist on the RS23e 2interface PCB.
I would then be inclinend to fidn some whay of cotnrolleing the TxD RS232
input ot the interface. Either by using the 'break' bit in the PC;s
serial chip or by disocnnecting that line from the PCB and using jumpers
to pull it high or low. WIth it low (-ve votlage), there should be 20mA
or some fliwiugn round the loop. 23mA is fine. WIth it high (+ve
voltage), there should be essentially no current flowing in the loop. If
you don't get this there is a hardware problem.
-tony