[GreenKeys] Current Loop to RS-232 Conversion on ASR33

Sun Feb 10 18:09:31 EST 2013

I tried a loopback test on my Westermo converter and that worked fine when looping back from PC back to PC through the current loop. I tried looping back from the TTY side via the RS232 interface and there was no echo.

I then tried the battery test you mentioned and unfortunately that failed to work too.

I think the problem must be inside the 33. L

Regards

Rob

From: Teletypeparts [mailto:teletypeparts at aol.com] 
Sent: 10 February 2013 22:28
To: r.jarratt at computer.org; greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Current Loop to RS-232 Conversion on ASR33

Rob,

I have heard of the interfaces being bad but I am afraid I have exhausted my limited knowledge of interfacing.  Some of the guys on Greenkeys may be able to help you.  

If you want to eliminate the 33 as a problem completely do the following:

Short together 4 and 6 and apply a 9 volt batt to 3 and 7.  Again, if it runs open reverse polarity on 3 and 7.  Do the test in Line mode.  After you have the 33 running closed, type a few characters.  If they print OK the trouble is not in the 33.  I still dont recommend leaving the batt on very long.  A minute or so.  It may not harm anything, but just to be sure keep the time short.  

What you have done is make the 33 half duplex (2 wire) and tested it in line mode.  I do that all the time here to check out the 33 in Line mode.  I also use a test generator, but before I had that I used the 9 volt batt test.  

Wayne

-----Original Message-----
From: Rob Jarratt <robert.jarratt at ntlworld.com>
To: 'Teletypeparts' <teletypeparts at aol.com>; greenkeys <greenkeys at mailman.qth.net>
Sent: Sun, Feb 10, 2013 4:28 pm
Subject: RE: [GreenKeys] Current Loop to RS-232 Conversion on ASR33

I do understand about the echo that is needed to get characters to print in line mode. I am using a terminal emulator on the PC to receive and display the characters I type on the ASR33, and I type characters into the terminal emulator to send them to the ASR33 in the hope they will be printed. The emulator successfully displays the characters I type into the ASR33, but the ASR33 does not print the characters I type into the emulator, despite the Westermo LEDs showing activity which suggests it is receiving the characters from the emulator and sending them on to the ASR33.

I had to re-read about start bits because I had forgotten about them. I think start bits are always present, so there is no setting to control them. So basically, I think I am already using 1 start bit, 7 data bits, one parity bit and 2 stop bits, but nothing is getting printed. I am using a line speed of 110 bits/sec, which in this case corresponds to 110 baud too.

I suppose the Westermo converter could be faulty and not sending data out to the ASR33. Unless anyone has any other suggestions I’ll try a loopback test on the converter to see if it is working.

Regards

Rob

From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net <mailto:greenkeys-bounces at mailman.qth.net?> ] On Behalf Of Teletypeparts
Sent: 10 February 2013 18:20
To: r.jarratt at computer.org; greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Current Loop to RS-232 Conversion on ASR33

Rob,

Read on as I put something important below.  

I guess the voltage doesnt need to be 80 or more.  It may run OK with less.  The norm is 20 MA at 120 VDC.  

I dont know anything about the Westermo, but I know that when you send to the computer from the 33 the computer needs to be programmed to echo back to get a copy on the 33.  

When you type in Local, the print is pretty much instant.  When you type in Line thru the computer you will see a slight delay, not much but enough to notice.  

The 33 doesnt care about parity on its receive side, it will print whatever the bits from 1 to 7 are.  The 33 is 110 baud, not sure what that converts to in computer language.  

Oh and this is important, you need a START bit which is always a zero or a space or no current which all terms seem to be interchangeable.   The rest you got correct.  Its one start bit, 7 data bits, one parity bit and 2 stop bits.  The 33 will run with 1 stop bit, but its normal to give it two when you can.  

If you put the START bit in, I bet it will work.  

Wayne

-----Original Message-----
From: Rob Jarratt <robert.jarratt at ntlworld.com>
To: 'Teletypeparts' <teletypeparts at aol.com>; greenkeys <greenkeys at mailman.qth.net>
Sent: Sun, Feb 10, 2013 5:36 am
Subject: RE: [GreenKeys] Current Loop to RS-232 Conversion on ASR33

I made some great progress, with one last hurdle to overcome.

I configured the Westermo to be in active mode (ie supply current) on both its receive and transmit sides). This cured the problem with the RD light on the Westermo being on all the time and it stopped the chattering too.

I connected it up to a terminal emulator running on my PC. When I press a key on the ASR33 the corresponding character appears on the terminal emulator. However, when I try to send characters from the terminal emulator to the ASR33 nothing happens. I am using 110bps, 7 data bits, even parity and 2 stop bits.

As the chattering has stopped, does this mean that the Westermo’s transmit side is set up correctly now?

Am I using the right serial port settings?

Could the Westermo not be outputting the right voltage? It is a 20ma interface, but the manual does not say anything about the voltage which you say needs to be 80V or more.

What else might prevent the characters from being printed out?

Regards

Rob

From: Teletypeparts [mailto:teletypeparts at aol.com <mailto:teletypeparts at aol.com?> ] 
Sent: 09 February 2013 20:30
To: r.jarratt at computer.org; greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Current Loop to RS-232 Conversion on ASR33

Rob,

The 33 should chatter or run open when in Line mode and not connected to anything.  The reading of a short is normal as on 3 and 4 you are basically looking at a switch, the distributor of the 33.  Same reading on 6 and 7 as I recall.  

Neither side of the 33 generates any current.  The receive wants 20 MA at 80 volts or more and the send will accept whatever the interface gives it, usually 12 or 15 volts.  

A quick test you can do is wire a 9 volt batt to 6 and 7 while in line mode.  You have a 50/50 chance of getting the polarity correct.  If still running open, reverse the leads.  The 33 should run closed.  Dont leave the batt connected for a long time.  You have now verified the 33 will run closed in Line mode and you need to proceed with the adapter stuff of which I know very little.  

I believe the Send side of the Westmo should be set to active to generate 20 MA current for 6 and 7 of the 33.  

Dont know about the Westmo receive side.  

Hope this helps,

Wayne

-----Original Message-----
From: Rob Jarratt <robert.jarratt at ntlworld.com>
To: greenkeys <greenkeys at mailman.qth.net>
Sent: Sat, Feb 9, 2013 7:22 am
Subject: [GreenKeys] Current Loop to RS-232 Conversion on ASR33

I emailed this list a little while ago about getting my ASR33 to talk RS-232. I have since sourced a Westermo MA-21 current loop converter.

I have noticed however, that the ASR33 will “chatter” in line mode regardless of whether the current loop wires are connected to anything or not. When I plug it into the Westermo the RD light on the Westermo comes on even if the ASR33 is disconnected from the mains. The RD light is supposed to come on when it is receiving data from the line interface (manual here if you want to see it: http://www.westermosales.com/pdfs/MA-21manualENGSEDE.pdf), which makes no sense if the ASR33 is not even connected to the mains.

I have set both the receive and transmit side of the converter to passive mode for now (ie they don’t generate the current), but I think the receive side of the converter will need to be set to be active so that it generates the current because the drawings of the ASR33 suggest that the ASR33 receive side generates current but the transmit side does not. In fact all the switches are set to the factory settings at the moment.

I tried switching the receive side of the converter to active and when I switched on the converter (with the ASR33 disconnected from the mains) the RD light did not come on this time.

I have also noticed that with the ASR33 disconnected from power the screw terminal pairs 3&4 and 6&7 appear to be shorted (ie 3 shorted to 4, 6 shorted to 7). I suspect this may be expected but I am not sure.

I have wired screw terminals 3&4 on the ASR33 to the Receiver side of the Westemo, and 6&7 to the Transmitter side.

I wonder if there is a fault in the ASR33, should it chatter in line mode when it is not connected to anything?

Which side (if any) of the Westermo should be set to active to generate current?

Regards

Rob

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