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

[Alf Fisher]

Current Loop to RS-232 Conversion on ASR33Hi Rob,
I'm not familiar with the MA-21 but I can tell you that with the ASR33 switched to Line, it should be quiet apart from the humming of the motor.
You wont be able to tell what is going on with motor power disconnected.  The fact that it is chattering, means that you have no loop current. There needs to be loop current on the receive side provided by your converter and it has to be the correct polarity in order to switch the transistors on and hold the electromagnet energised in the Mark position.

Others on here may have more knowledge of ASR33 installations than I do.  I suspect in some installations, loop current was provided by a power unit in the machine base although this is not been the case with the few machines I have seen.  

On the keyboard / tape reader transmit side, I think that current should be provided as the keyboard contacts merely interrupt the current flow with a series of pulses when a key is pressed.  It will depend on what your MA-21 requires by way of input but I think this current should be provided by your MA-21 but I don't think polarity is important in the transmit mode because the transmitting mechanism only consists of contacts that open. 

Hope this helps.

regards,
Alf
Potton,
Beds.
UK.

  ----- Original Message ----- 
  From: Rob Jarratt 
  To: greenkeys at mailman.qth.net 
  Sent: Saturday, February 09, 2013 12:21 PM
  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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