20 Sep
2013
20 Sep
'13
11:35 p.m.
On 9/20/2013 1:04 PM, Tony Duell wrote:
I have looked at the PDP8/L maintenance manual on
bitsavers. If the
scheamtic of the teletpye interface in Vol 1 is complete and accurate
(and I think it is), then it is clearly an active device and is actualyl
quite simple. I don't think the problem is there.
The docuemant on the RS232 adapter also incudes a schematic. I would work
through that. My first check would be on the LEd side of the optoisolator
in the PC->PDP8/L circuit -- that is the one where the LED is being
driven by the PC (RS232 interface). First check that the LED current
changes when you send breaks. If it doesn't you need to troubleshoot the
RS232 receiver, LED driver, etc. If it does, then you need to
troubleshoot the circuit on the transistor side of that optoisolator. It's
only a couple of transistors.
Thanks. I didn't have an incredible amount of time this evening, but I
was able to verify that the LED side of the optoisolator is getting
current (about 9mA) when a Break is sent from the PC (drops to 0
otherwise), so it looks like the idle state for the optoisolator is "off."
On the other side of the isolator, if I use your "dirty trick" and short
out the output-side of the isolator to force it on, the current loop
current only drops to about 20mA (similar to when I send a break from
the PC, which at least matches the behavior I'd expect currently). So
the RS232 side seems ok, and it looks like the fault is on the other
side of the optoisolator.
Unfortunately, everything's surface-mount components on this thing and
at the moment I don't really have good tools for working with stuff this
small, so I may pick up a few things (small tweezers, finer probes,
etc.) so I can more carefully test things this weekend.
Thanks again for the advice.
Josh
-tony