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

Sun Feb 10 05:36:54 EST 2013

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] 
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> 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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