PDP-8/L current loop -> RS232 help

from exceeding its power and burning out.

From: hilpert at cs.ubc.ca Subject: Re: PDP-8/L current loop -> RS232 help Date: Sat, 21 Sep 2013 15:41:20 -0700 To: cctalk at classiccmp.org 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