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

[Dave]

On 20/01/2013 18:29, John Nagle wrote:
> 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.

It was rescued from a dump truck by a friend of mine. Rob persuaded me 
he was going to use it. I didn't  know any thing about its provenance....

>> 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
>
>
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