[GreenKeys] Problem with Current Loop to RS-232 Converter on ASR33

[Clay Archer]

Looks like a basic level-translator with one transistor and a few diodes and
resistors.   It looks like it is using the Data Terminal Ready pin 4 of the
DB25 connector to obtain its power.  DTR usually sits at about -12V.   The
transmit data from the computer drives the base of the transistor to drive
the receive data current loop of the teletype.  Transmit data from the
teletype drives the serial directly with diodes to clip the voltage.   It's
also possible a source of loop voltage may be provided by the teletype's
power supply.  There are five wires from the teletype, only four are needed
for the transmit and receive loops.

 

The problem with this design is that it doesn't meet the RS232
specification, which means that it may not work with some computers.  Some
computers provide +/- 5v signals on the RS232 port, or don't provide the DTR
on pin 4, which won't work with this design.  It also doesn't provide any
isolation between the teletype's circuits and the computer which could
result in a damaged serial port on the computer, or worse.   If you decide
to make this one work be careful, make sure to have the computer and
teletype plugged into the same grounded outlet.

 

If you have a volt meter you could check and see if there is any voltage on
pin 4 (pin 7 is ground, 2 & 3 is transmit and receive) and any voltage on
the five wires from the teletype.  If the teletype stops chattering with the
DB25 connector not plugged into anything, then it's getting its loop power
from inside the teletype.

 

The standard current loop pinout on the screw terminals on the back of the
ASR33 are:

3 + Loop In

4 - Loop In

6 - Loop Out

7 + Loop Out

 

Which is actually backwards, the Teletype receives data on 6 & 7, and
transmits data on 3 & 4.  I don't know why Teletype labels them that way.
You should trace the five wires from the converter box and see if they go to
the screw terminal strip on the back.  Also to see where the 5th wire goes.
If you get a different RS232 to current loop converter you will need to know
if the wiring in the teletype has been modified to make this interface work.
You can find schematics for the ASR33 on bitsavers.org.

 

I have some schematics of some converters, or I could give you some links to
ones you could buy.    You could make you own with proper opto-isolators and
such for about $15 if you know how to use a soldering iron, or buy one for
about $80.   You could also possibly make the one you have work by adding a
battery and a resistor.

 

Let me know how it goes, I like to see these things work.

 

By the way, check your print hammer, the little arm that sits between the
ribbon spools that smacks the print head (cylinder with the letters on it).
If the rubber is worn or missing you will ruin you print head.

 

Clay

 

  _____  

From: Rob Jarratt [mailto:robert.jarratt at ntlworld.com] 
Sent: Sunday, January 20, 2013 3:07 AM
To: 'Clay Archer'; r.jarratt at computer.org; greenkeys at mailman.qth.net
Subject: RE: [GreenKeys] Problem with Current Loop to RS-232 Converter on
ASR33

 

I opened it up and there are no chips in it at all. This is what it looks
like: http://sdrv.ms/13TtsbR

 

Looks like I will need to source a converter. Is there anything in
particular I should be taking into consideration?

 

Regards

 

Rob

 

From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Clay Archer
Sent: 19 January 2013 22:24
To: r.jarratt at computer.org; greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Problem with Current Loop to RS-232 Converter on
ASR33

 

It sounds like the current loop side is going into "open loop".   Does your
RS232 converter have a model number or any identifiable markings, or is it a
"home brew" box?   If you could open it up and post a photo that would be
helpful.  I would expect to see some opto-isolators and an RS232 driver
chip.

 

Most RS232 to current loop converters require an external power supply to
provide the loop current, and sometimes to power the RS232 side.   Sometimes
there is an extra connector on the "box", or it may expect the extra
voltages through the DB connector.

 

Clay

 

 

  _____  

From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Rob Jarratt
Sent: Saturday, January 19, 2013 11:31 AM
To: greenkeys at mailman.qth.net
Subject: [GreenKeys] Problem with Current Loop to RS-232 Converter on ASR33

 

The ASR33 I got came with a little box that I assume to be a Current Loop to
RS232 converter.

The ASR33 behaves rather oddly when I plug the RS232 connector into
anything. When the connector is not plugged into anything the ASR33 ticks
over as I believe it should. However when I plug the connector into a
socket, even if the socket is not connected to anything else, the ASR33
starts acting a little as if it receiving characters, the cylinder with the
characters embossed on it "vibrates" but does not print anything. If I plug
it into my breakout box, with the other end not connected to anything, it
acts in this way and I notice the Rx light come on, which of course it
shouldn't as nothing is being sent to it. It is almost as if there is a
short to the Rx line. I have checked the connector for shorts but there
don't seem to be any.

Has anyone ever come across behaviour like this?

Regards

Rob

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