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

[John Nagle]

On 1/20/2013 2:07 AM, greenkeys-request at mailman.qth.net wrote:
> Date: Sat, 19 Jan 2013 18:31:05 -0000
> From: "Rob Jarratt" <robert.jarratt at ntlworld.com>
> 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.

    More detail on the provenance of the device would be useful.
> 
> 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 believe it 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, it
> acts 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 across behaviour like this?

     RS-232 is supposed to have an input that swings from -12V to +12V.
Most modern devices (last 20 years or so) accept 0 to +5V as input.
Some older devices may not do that.  Also, RS-232 inputs are supposed
to have some load on them; an input resistance of 3K to 7K is specified
for RS-232 inputs, so that random electrical noise isn't sensed as
data.

     Does your device work when attached to some proper RS-232 data
source?  If the only problem is that it behaves badly when unplugged,
just put a 5.6K resistor across the input to sink the noise.

Read this: http://www.arcelect.com/rs232.htm
And this: http://www.maximintegrated.com/app-notes/index.mvp/id/723

					John Nagle