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

[Rob Jarratt]

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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