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

Mon Jan 21 17:56:12 EST 2013

Hi Rob,

 I recall pin 4 is Request to Send, pin 20 is Data Terminal Ready. It (theoretically) should not be putting out any voltage on pin 4 untill you try to send something. If the interface is putting a voltage on RTS, perhaps your PC is putting a signal on pin 3, albeit a 5v signal. I seem to recall not all PC serial connections were RS-232 friendly. Maybe try using your break-out box as a quasi crossover connection:
connect pin 4 to 5  (RTS to DTS)
connect pins 6 to 8 and 20 (DSR to DCD and DTR)
All these connections should be on the PC side ONLY.
This takes care of all the handshake signals, leaving only TD and RD connected. As a last resort, cross RD on the PC side with TD on the RTTY side, and vise versa. While this doesn't address the possibility of a voltage mass-match, it should eliminate other incompatibility problems.

Mike - N9ODM

ASCII and ye shall receive.

________________________________
 From: Clay Archer <carcher at parkcity.net>
To: r.jarratt at computer.org; greenkeys at mailman.qth.net 
Sent: Sunday, January 20, 2013 9:17 PM
Subject: Re: [GreenKeys] Problem with Current Loop to RS-232 Converter on ASR33

Problem with Current Loop to RS-232 Converter on ASR33  
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 believeit
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, itacts 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 acrossbehaviour
like this?
Regards
Rob
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