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

[Rob Jarratt]

> -----Original Message-----
> From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-
> bounces at mailman.qth.net] On Behalf Of John Nagle
> Sent: 20 January 2013 18:30
> To: greenkeys at mailman.qth.net
> Subject: Re: [GreenKeys] Problem with Current Loop to RS-232 Converter on
> ASR33
> 
> 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? 
[Rob Jarratt] 


No. I used a breakout box to try to get the signals right, but no matter
what I did it would not work.


 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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