RS485 Testing

BrendanDixon · Apr 26, 2013

Hi All,

We have been having problems with the communication on a sign board it uses RS485 comunications to communicate between the computer and the sign board. The cable of about 30m tests fine with a multimeter but when I try and communicate nothing will happen. I am trying to make a tester using 2 rs232 to RS485 converters and a program called access port with a RS232 loopback plug on the one converter. but I cannot seem to be able to get this to work as I send text but do not get it back. I am not using the suspected faulty cable.

I have a feeling that is is something to do with the loopback plug as I have pins 2 and 3 bridged and pins 7 and 8 bridged.

danw · Apr 26, 2013

1) If you are using two 232/485 converters, then the native communications must be RS-232 on both the PC and sign.

If you put the PC closer to the sign and connect it directly via RS-232, does it work?

Which 232 terminals or pins are needed for this to happen? Just Tx, Rx and signal ground?

2) A loopback plug is a test fixture to prove that what goes out from one end can come back again and be read by the send, given the right test setup. A loopback plug is a substitute for a device on the end. A loopback plug is not used in a working mode, it is only used in a text mode.

A hardwired Loopback test applies to RS-232 only. A 2 wire RS-485 link will not respond to a hardwired loopback test because the tester shorts the (+) and (-) pins together and makes 485 dysfunctional.

3) Unless you tell us what pins 2 and 3 and 7 and 8 are for both ends, it's a mystery. I'm not looking over your shoulder.

4) It's the cable capacitance that limits RS-232 to a 50 ft (15m) limit, but I've lucked out and done 100+ feet (30m).
http://electronicdesign.com/what-s-...e-between-rs-232-and-rs-485-serial-interfaces
It might pay to try directly wiring 30m via RS-232 and see if it works. It will be more subject to electrical noise.

a062549 · Apr 26, 2013

danw said:
1) If you are using two 232/485 converters, then the native communications must be RS-232 on both the PC and sign.

If you put the PC closer to the sign and connect it directly via RS-232, does it work?

Which 232 terminals or pins are needed for this to happen? Just Tx, Rx and signal ground?

2) A loopback plug is a test fixture to prove that what goes out from one end can come back again and be read by the send, given the right test setup. A loopback plug is a substitute for a device on the end. A loopback plug is not used in a working mode, it is only used in a text mode.

A hardwired Loopback test applies to RS-232 only. A 2 wire RS-485 link will not respond to a hardwired loopback test because the tester shorts the (+) and (-) pins together and makes 485 dysfunctional.

3) Unless you tell us what pins 2 and 3 and 7 and 8 are for both ends, it's a mystery. I'm not looking over your shoulder.

4) It's the cable capacitance that limits RS-232 to a 50 ft (15m) limit, but I've lucked out and done 100+ feet (30m).
http://electronicdesign.com/what-s-...e-between-rs-232-and-rs-485-serial-interfaces
It might pay to try directly wiring 30m via RS-232 and see if it works. It will be more subject to electrical noise.

USed some CAT5e for good distances for the 232 stuff also. Easy to find in a pinch too.

RS485 Testing

BrendanDixon

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danw

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a062549

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