21 Sep
2013
21 Sep
'13
8:16 a.m.
When U1 turns on, it shunts the bias for the output
transistors and turns them off ( working correctly ).
If 20mA is flowing, something isn't right. Q1 and Q2
are connected as a darlington pair. Q1 provides the current
boost for Q2. With the opto turned off, you should see
about 1.2 volts on the base of Q1 relative to the emitter
of Q2.
Q2's base should be about .6 volts.
Dwight
From: dkelvey at hotmail.com
To: cctalk at classiccmp.org
Subject: RE: PDP-8/L current loop -> RS232 help
Date: Sat, 21 Sep 2013 06:52:29 -0700
From: derschjo at gmail.com
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.
You say, it only drops to 20mA. What is it dropping from?
If it is still 40mA you have multiple problem.
As has been said, the 33 is suppose to be a passive end
but it can be configured as the active end by someone before
you. If you have 40mA something is not right. It is not transistors
turning on, it is an additional supply or wrong resistors.
With the output of the opto shorted, the loop should
go to 0 mA ( actually about 60 uA because of the 200K
resistor ).
If as you say, it only drops to 20mA something on the
interface should be getting warm. 12V at 20mA is about
1/4w for small surface mount parts you should be able
to feel that with your finger.
Dwight