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Receiving either RS-232 or RS-485

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DSGarcia

New Member
How might I construct a circuit to receive serial data (for a PIC) from either RS-232 or RS-485 without it mattering the type of the source?
Thanks,
Dale
 

Papabravo

Well-Known Member
Have a transceiver of each type. Send the identical data out on both transmitters. Take the two receivers and combine the outputs in such a way that if one of the receivers is idle, the other will be able to make the Rx line to the PIC beat up and down.

Will that meet your requirements?
 

DSGarcia

New Member
PB,
I will only have one external 4-pin connector (or 3-pin if I can ignore one of the RS-485 lines--read on for explaination). That single connector will be fed by either a RS-232 signal or a RS-485 signal but not both at any given time.

If I have both types of receivers, I could OR them together, but I was not certain how it would work with one receiver always receiving slightly out-of-spec voltage levels.

Is there another method I can use using one receiver but will not be hurt by voltage levels from both types of transmissions? The distance is less than 100 feet and I can use 9600 BAUD so I think I can ignore one of the RS-485 lines if it would help.
Thanks,
Dale
 

Leftyretro

New Member
PB,
I will only have one external 4-pin connector (or 3-pin if I can ignore one of the RS-485 lines--read on for explaination). That single connector will be fed by either a RS-232 signal or a RS-485 signal but not both at any given time.

If I have both types of receivers, I could OR them together, but I was not certain how it would work with one receiver always receiving slightly out-of-spec voltage levels.

Is there another method I can use using one receiver but will not be hurt by voltage levels from both types of transmissions? The distance is less than 100 feet and I can use 9600 BAUD so I think I can ignore one of the RS-485 lines if it would help.
Thanks,
Dale

Well the problem is that RS-232 is bipolar single ended signal referenced to a signal ground pin and RS-485 is bipolar differential ( 2 pins) with no signal reference. They require drastically different receiver circuitry. I'm not saying there is no way to fool some kind of receiver circuit into responding to both types but I don't think it would be reliable even if possible. At the least there would be a need for a separate ground wire connection from the device sending the RS-485 to the receiver circuitry I think.

Lefty
 
Last edited:

Papabravo

Well-Known Member
You can try to do anything your heart desires. That doesn't mean it will work. I think your efforts are unlikely to be successful.

I suppose there might be some kind of discrete circuit to do the job, but I'm not going to waste a minute trying to figure one out.
 

DSGarcia

New Member
Lefty,
I can provide a reference ground with the RS-485 signal. Let me think about your comments.

PB,
Thanks, I knew this would be a challenge; that is why I posted to see what the thoughts on this subject were.

Thanks,
Dale
 
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