22 Sep
2013
22 Sep
'13
12:04 p.m.
On 2013 Sep 22, at 10:31 AM, Josh Dersch wrote:
For the record, the measurements you asked of me
earlier are now:
- W076 with pins 3,4 open (no external connections to 3,4,
measured relative to the PDP-8 ground):
- pin-4 should = ~ +5V
With or without -30V connected, I'm getting 4.6V.
Great, the W076 looks good, and there's logic-level switching at
point E.
(I presume that's actually -0.6V relative to GND at pin-E that you're
measuring.)
I'd turn your attention now to the converter and check it in
isolation, disconnected from the loop.
Wire it up as suggested earlier with a 10-15V supply and make sure
it's working properly, although I was a little off with the levels to
expect. Here's an amended version:
- Connect T- to 0V/GND, and T+ to a V+ = (+10 to +15V) through a
1K-or-so current-limiting resistor,
- T+ should show ~ +1.5V and V+ switching with the RS-232 signal-in.
- if that works, try boosting the V+ to +20 to 24.
It looks like D1 and D2 are there to provide some minimum voltage
drop so the 6N135 always has some Vcc.
( ref: http://www.bb-elec.com/Products/Datasheets/r2_Website-
Only_0812DS.pdf )
What Dwight was saying earlier about it likely not being damaged
would be true for a common 6-pin type optoisolator, unfortunately the
6N135 is an 8-pin type with separate Vcc connection, so it would have
seen the excessive 35V directly across it (pins 8 and 5). I don't
know why they used such a device, the thing isn't going to be working
at really high speeds. When I designed a little current-loop adapter
a few months ago, I was targetting 60mA and 20mA loops and had to
make sure it would tolerate higher voltages (over 70 or even 100V),
but still just used a common-as-dirt type opto-isolator.
I don't expect the current level is an issue, I don't see any problem
with it dealing with 40 or more mA, the 3904s are rated up to 200mA
and the darlington pair should be enough gain to keep them from
saturating.
- pin-3 should = ~ -15V
With -30V connected, I get -34.6V, with it disconnected I get -16.5V.
- pin-E should = ~ +5V
With or without -30V connected, I get 4.6V.
- W076 with pins 3,4 shorted (and no other connections to 3,4):
- pins-3,4 should = ~ -(15-2*0.6) = ~ -13.8V
With or without -30V connected, I get -12.7V
- pin-E should = ~ -0.6V (one conducting
diode drop)
With or without -30V, this is 0.6V.
I think you should figure out what's going on with the W076 before
thinking about modifying the converter. Should try to get the
expected behaviour and measurements out of the W076 without the
converter involved. If that resolves as calculated, the lower V of
20V ( =5-(-15) ) may still be pushing it for the 6N135 but it may
work.
So it looks like things are correct (or at least very close) now.
The converter still doesn't appear to be working, however.
Although, according to the docs it was made to work with 20mA loops
and mentions nothing about higher currents (the main page for the
device does state "Many models can be modified for higher loop
currents and voltages" which I suppose is always true as long as
they don't coat the circuits in a blob of epoxy... :) )
Thanks everyone for the help. I think it's clear that I need to
spend some time learning more about analog electronics -- I'm
getting pretty decent at debugging digital stuff, but the analog
domain is clearly my weakness...