21 Sep
2013
21 Sep
'13
3:41 p.m.
I'm looking at the RS-232 converter schematic:
http://www.bb-elec.com/Products/Datasheets/r2_Website-
Only_0812DS.pdf
and this PDP-8/L schematic:
http://bitsavers.trailing-edge.com/pdf/dec/pdp8/pdp8l/DEC-8L-HR2A-
D_8Lschem_Feb70.pdf
power supply: page 25
W076: page 37
and
http://www.fairchildsemi.com/ds/6N/6N136.pdf
(So all of the following is derived from examining the schematics,
not previous measurements. I had to redraw the W076 diagram so it
makes a little more sense.)
One thing you might check is whether the 'optional' -30V is present
on the W076, it appears it can be supplied internally or externally.
The power supply schematic indicates it is produced by the 8/L
supply, whether it makes it to the W076 will require examination.
With the -30V the 'open-state' loop voltage would be +5-(-30)=35V,
which exceeds the max specs for the 6N135 opto-isolator in the
converter. Could be sending it into breakdown/conduction or damaged
the converter.
Try removing the -30 if it was present. Without the -30 it appears
the W076 loop voltage should be +5-(-15)=20V (diode switching of
supply) which the converter might be happier with. The converter
design appears to me to be a little on the weak side for V specs.
If the -30V was present, you might try checking the converter
disconnected from the PDP-8.
- Connect T- to ground, and T+ to a V+=(+5 or +12) through a 1K-
or-so resistor.
- T+ should show ~ +0.2V/V+ switching with the RS-232 signal-in.
Another suggestion could be to confirm the W076 is working properly.
If I have things more or less right, without the -30 supply and the
loop disconnected:
- W076 with pins 3,4 open (no external connections to 3,4,
measured relative to the PDP-8 ground):
- pin-4 should = ~ +5V
- pin-3 should = ~ -15V
- pin-E should = ~ +5V
- W076 with pins 3,4 shorted (and no other connections to 3,4):
- pins-3,4 should = ~ -(15-2*0.6) = ~ -13.8V
- pin-E should = ~ -0.6V (one conducting diode drop)
It looks like the receive-loop current from the W076 will be around
43 mA (a current meter between pins 3-4 for example) which seems odd,
but that's what my calcs say, unless I've misinterpreted something in
the schematic. But it matches what you've been seeing doesn't it?
On 2013 Sep 20, at 11:35 PM, Josh Dersch wrote:
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