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vrf RS485 Loopback test

Question asked by VRFuser on Jun 13, 2006

I've done a lot of work with MSComm and other "outside" serial port drivers. I also use RS485 extensively in the same manner you appear to intend. It seems that testing the 485 port output as you outlined will work perfectly - use two serial ports; talk on one and listen on the other. The MSComm drivers don't need to know anything about 485 so I'm not sure what help you really need.

I tend to use 2-wire 485 and run it out from the control computer, around the block, and back to the control computer. I put a 232-to-485 converter on both ends and attach them to different serial ports. The sensors are all connected across the pair of wires and they respond to their addresses when issued by the "master" serial port. The nice part comes when considering what the second serial port is doing: monitoring that port allows me to see all traffic on the loop, both the sensor addresses and their responses. If there are problems, such as might occur if two sensors respond to the same address, or a sensor continues to talk while the master is trying to issue a new address, the second port will clearly show the result (probably in the form of garbled text). Finally, because its at the end of the "loop", the second port provides a way the assess the loop's physical health.

Let me know if I can provide any specific help on using MSComm for your application.


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