Olin Lathrop wrote:
> Josh Koffman wrote:
>   
>> Hi all. I'm working on a board that has a specific use at the moment,
>> but I'd like to leave things a bit open for other future uses as well.
>> At the moment, there will need to be a connector that talks to a
>> device via RS232. It's low speed (9600bps) and short distance (under
>> 10 feet, likely under 3 feet). To keep my options open for the future
>> (and to aid my design), I'd like to use RJ45 connectors to carry the
>> RS232 signal. There is a standard for this (EIA-561, mentioned here
>> about 25% of the way down the page, but I've seen it elsewhere as
>> well) so I feel ok setting it up. I believe that the original intent
>> was to use telephone style cable with these RJ45 connectors. What
>> would happen if I used straight through CAT5 ethernet cable?
>> Electrically I think it'd be ok, and since RS232 isn't a differential
>> signal the pairs shouldn't have to be matched.
>>
>> How wrong am I?
>>     
>
> Seems reasonable to me.  Just make sure that each signal (receive, transmit)
> is on its own twisted pair with the other wire of the pair being ground.  In
> other words, the minimum RX/TX lines will use up half the CAT5 cable.
>   
however many offices use 9,600 or 19,200 RS232 with only one earth wire 
and 4 or 6 signals over CAT5 structured wiring, so possibly up to 100m.


This to my mind is most common scheme. The RI pin on DB9 is not 
connected. A PC can't generate it. AFAIK only Modems generate it (when 
the phone is ringing!) This is also why many OS you can interrupt on 
both directions of change on the other Handshake pins but only interrupt 
in one direction for RI as it's an event only prior to link establishment

RJ 45 http://pinouts.ru/SerialPorts/rs232d_pinout.shtml
http://pinouts.ru/SerialPorts/Serial9_pinout.shtml

If a modem is connected over CAT5, then pin 1 is RI on CAT5/RJ45, and 
RJ45 pin 1 is connected to pin 9 instead of  pin 6 on DB9

There is an EIA table of distances and speed. It assumes a particular 
wire type that is just bundled multi-core with a single overall screen.

Olin's custom twisted pair scheme of ground on each half of each pair is 
only needed either for 115k or longer distance.


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