22 Sep
2013
22 Sep
'13
10:31 a.m.
On 9/22/2013 2:53 AM, Brent Hilpert wrote:
On 2013 Sep 21, at 10:40 PM, dwight elvey wrote:
Yes, -15V is supplied (at pin B, nothing at point K as expected given
the docs). But -- for some reason this wasn't making it to the output
lug 3. I had replaced D7 and while I had taken care to orient the diode
correctly and the solder joint was visually good, I was getting no
connectivity from the anode leg to the rest of the circuit. I
resoldered it and now -15V is showing up at 3.
Now, with or without -30V supplied I get ~39mA in the loop (drops to
28mA on break).
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.
I'm looking at the converter schematic but it
is onpage 37?If it is a
8L, I figure 20 mA.It is -30volts to +5V or 35V.R1 and R2 are 1K and
750 or 1750 ohms.35/1750 = .02 amps or 20mA. Point N is pulled to
-10v when there is 20mAflowing.One could clamp point P to -10V with a
10 voltzener to ground. That would still get the 20mAwhen the
contacts were close but limit the voltageacross the contacts ( RS232
converter ) to 15Vinstead of 35V.A half watt zener would work here as
it is only0.2Watts to deal with.I hope this makes sense.Dwight
(Dwight, just for clarity, I'm referring to the the RS-232->current
loop dongle as the converter, the W076 interface card being on page 37
in the 8/L schematic.)
Some more of the reasoning from the earlier examination of the schematic:
- When the loop (3,4) is open: pin-4 is pulled up to +5 through
R2(750), pin-3 is pulled down to -30 through R1(1K) and D2 *if the -30
supply is connected to the W076*. However if the -30 is not connected,
pin 4 is (should be) pulled down to -15 through now-conducting D3 and
D7. (D3 and D7 are reverse biased/off if -30 is connected).
- When the loop is closed (3,4 shorted), -V (15 or 30) has two paths
to flow: to +5 through R2 and to ground through R4 (750) and D4 (D4
now forward biased and clamping the negative V (intended so point E to
the TTL logic doesn't go too negative).
I get the 43mA from the sum of those two paths (25mA to +5 plus 18mA
to GND) when -30 is disconnected. From my calcs, when the -30 is
connected the current is actually lower, despite the higher V, because
of the higher impedance through R1(1K).
-
Josh, the measurements you presented make sense for the -30-connected
case, but not for the not-connected case. They suggest that the -15 is
not present. Check whether -15 is supplied to the card (I expect it is
as IIRC you said the 8->PC direction was working, should be point B
but note doc also mentions point K).
Without the -30, -15 should be making it through D3 and D7 to pin-3
(to test, might need a slight load to conduct when open-loop). You
said you replaced a broken diode earlier, might the new diode be
installed backwards? If it's D3 or D7 it would explain the lack of -15
at pin-3 and would also (counter-intuitively as per above) explain the
reduction in the current (~40mA down to 23). - 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...
- Josh