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Current Loop Adapter wiring

NeXT

NeXT

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I built a 20ma current loop adapter from this schematic but I'm having trouble getting it to work.

For testing I have it attached to a Hayes Smartmodem which will handle 110 baud in command mode.
The teletype is set in half duplex 20ma mode. Cabling is arranged so the Rx terminals on the teletype connect to the Tx block on the adapter which at the least should let the selector solenoid energize when everything is turned on but it doesn't. The teletype chatters away as if the loop is open until I bridge the terminals with something like an ammeter (which reads only -11.7ma on the adapter's TX pins). The adapter is powered currently from a DC bench supply and is drawing just over 20ma so it's at least using a predictable amount of power.
 
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If you disconnect the adapter and just bridge terminals 6 and 7 (the Rx connections on the teletype) the solenoid engages so I'll assume the teletype is the current source.

Also, I don't think it changed much but I substituted the ICL232CP with a MAX232CPE as they seem to be the same thing.
 
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NeXT said:
If you disconnect the adapter and just bridge terminals 6 and 7
Click to expand...

OK, next thing to try is a resistor to the 5v with the ammeter in between that and the collector with the emitter to ground
and see if the optoisolator is on. Also, you've checked the polarity of the loop?
 
Is there any particular resistor rating you have in mind? I can test the total current draw of the 4100 by lifting pin 8 and putting an ammeter between +5 and the pin, which reads 6.8ma.

I'm taking the notation of polarity from the manual for the model 33's section on the UCC6. Terminal 6 is RxD- and terminal 7 is RxD+ which should route to pins 4 and 3 respectively on the 4100.
 
NeXT said:
Is there any particular resistor rating you have in mind? I can test the total current draw of the 4100 by lifting pin 8 and putting an ammeter between +5 and the pin, which reads 6.8ma.
Click to expand...

OK, that probably means the LED is on. You just need to make sure the positive side of the loop is on the collector of the optoisolator output.
If you can see current with the meter in series with the optoisolator and the tty, it should be working

/ ----- - meter + -------- tty+
|
\ ---------------------------tty -
 
Fifth time is a charm. :huh:
So apparently there was a mistake in my wiring I missed the first four times. Pin 6 of the MAX232 runs to the negative side of a cap, then to ground. I had the positive side of the cap wired to +5. I've fixed it but this didn't fix the problem either. I've gone over my teletype schematics again and I'm still equally confused. Completely ignoring the Tx lines for the time being, That I can find the Rx side should not supply loop power when in LINE mode, yet we can see it has power simply by bridging terminals 6 and 7 and observing the selector magnet engaging without any external source. I've mentioned this already and nobody has said this might be a problems so now I don't know if it IS or IS NOT a problem or if it's something else.
Going back to Tx, nothing. Nada. Poke the 4200's pin 7 with a logic probe while pressing keys on the teletype and you get nothing. What sense can I make of that? None. I can't make sense of any of the readings I've taken so far because I have nothing to compare to.
 
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Put a scope lead on pin 13 and then pin 12 of the
232 chip.
Send some characters. You should see activity going in
on 13. Then you should see it going out of pin 12.
Tinker Dwight
 
According to the spec sheet. the HCPL-4100 should conduct
if the input power is off.
You might try this to make sure your isolator is working.
Shut the bench supply off and the teletype should lock up.
You should also check the open circuit voltage of the teletype.
The isolator is only rated for 27V max. I don't recall what
the supply was in the ASR33's but it may be more than 27V.
Dwight
 
Okay, so several things to report.

1 - Serial connector was pinned wrong. Discovered it after I attached my RS-232 tester and found RxD and TxD were reversed. Now running through HyperTerminal and getting proper DTR and RTS status with DSR coming on with the adapter attached. I'll have to change that if I want to use a modem.

2 - I can hang my scope off the MAX232's pin 13 now and see a good 10v high/low state when you type. Pin 12 gives me a good 5v rise/fall and can follow it to pin 6 on the 4100 so the MAX232 is behaving.

According to the spec sheet. the HCPL-4100 should conduct
if the input power is off.
You might try this to make sure your isolator is working.
Shut the bench supply off and the teletype should lock up.
You should also check the open circuit voltage of the teletype.
Click to expand...
Doesn't matter which way I connect the wires from the teletype, nor if the adapter is on or off. It chatters away.
I don't know how to check the open circuit voltage of the teletype. The schematics say the LOCAL loop power is 50-70v but I don't see anything for LINE.

Edited: Went ahead and tried poking at the 4100 again. When I type I can see its current draw drop 0.04ma which might be the LED going off and on as it should, or it could just be electrons farting. Pins 3 and 4 (TX+ and TX-) give a diode reading of 1.307 one way and .534 the other but still give no dead short across which the teletype wants. I would assume if we used the wrong voltage we would of blown everything up.
 
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You didn't try turning off the DC supply to your adapter
but that may not help any.
The voltage of the teletypes supply is too high. It has most likely
smoked the 4100. You can measure the line by disconnecting
your adapter from the teletype and just measure across the leads.
If it is greater than 27 volts, it has most likely smokes the 4100.
You need to use an external supply with a lower voltage of about
24 volts. You need to up the current with a resistor to get the
needed ma.
What is the coil resistance of the tty? Also, don't reverse the leads
on your new 4100, without a protection diode or it will not likely like
it.
Dwight
 
Well, that's very wrong.

Noting polarity, I'm seeing -0.5v DC and 6.6v AC. Yes, I'm poking terminals 6 and 7. Absolutely nowhere in the receive loop circuit should there be AC power!
Uhh, looking at the schematics again (Here, time to offer them out so you can see what I see) I can't even see a place where AC could leak in.
I still can't say how exactly this might of killed the 4100 but I guess if Q1 on FS-1 was rugged enough the reason the machine only shuts up if I bridge the terminals is because it's seeing the closed AC loop as a SPACE.

The coil is actually isolated from the loop on the teletype by means of the Magnet Driver Circuit. The card and everything past it I know at least works in LOCAL mode as I had to replace a shorted diode a month ago and had to check over all of it. Local loop power is physically isolated from the external loop when you run in LINE mode using the mode control relay.
 
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NeXT said:
Well, that's very wrong.

Noting polarity, I'm seeing -0.5v DC and 6.6v AC. Yes, I'm poking terminals 6 and 7. Absolutely nowhere in the receive loop circuit should there be AC power!
Uhh, looking at the schematics again (Here, time to offer them out so you can see what I see) I can't even see a place where AC could leak in.
I still can't say how exactly this might of killed the 4100 but I guess if Q1 on FS-1 was rugged enough the reason the machine only shuts up if I bridge the terminals is because it's seeing the closed AC loop as a SPACE.

The coil is actually isolated from this on the teletype by means of the Magnet Driver Circuit. The card and everything past it I know at least works in LOCAL mode as I had to replace a shorted diode a month ago and had to check over all of it.
Click to expand...

That low a voltage, the 4100 may have survived.
You need to find what has been done in the wiring.
Do note, when you get it straight, the 4100 is rated at 27V MAX.
And for safety, put a diode across the 4100 outptut, reverse bias,
to protect it from accidental problems. 1N4002 would be fine.
Dwight
 
The AC volts should not be there.
Without the internal loop supply connected, you should see about
-0.5 to -1v someplace. With the internal supply, it would be
around 48VDC.
Way to high for the 4100.
Dwight
 
The entire FS-1 receive circuit operates on either -4.7 or -20v DC. You're referring to the FS-11 supply which is 48v AC (and powers FS-3 which then supplies 50-70v DC for local loop power) and physically disconnects from FS-3 by means of FS-4 when in Line mode.
That being said, the loss of DC and the gain of AC would make sense if a diode gave out and perhaps one half was being rectified, especially CR1 or CR2 but what confuses me is that the loop continues to work if you bridge terminals AND all the diodes I've checked on the 81821 circuit card are fine. PLUS, how the hell do you get 6.6v? I'm buried in a class project right now but I'll try and double check my voltages before the end of the week hopefully.
 
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