I had a customer who asked me to integrate Newport i800 panel meters into a machine control system where a PLC queried the meters for their displayed readings through the RS-485 port provided on the meters.
I spent many days trying to communicate via RS-485 with these meters with communications settings at 9600,8,ODD,1 with no success. Then I tried all different baud rates, again with no success. I was able to view the RS-485 signals with an oscilloscope, and they looked good going into the meters, but I got no response whatsoever.
I tried different meter addresses. I tried different attention characters at the beginning of the message and different termination characters at the end. I tested the wiring a dozen times, swapping the two RS-485 wires or not, connecting the shield at both ends or not, etc. I even connected a PC in place of my PLC and ultimately had both PC and PLC on the network.
In all cases, communications out from my PLC looked good, and the messages appeared correct on the monitoring PC, but still no communications. I could put any one of the meters in RS-232 continuous broadcast mode and see what appeared to be good signals coming out of the meter, but my PC terminal would still not read them.
I made several calls to Newport tech support people, who made very good generic suggestions about termination, trying different baud rates, etc., but ultimately they provided no information that led to solving the problem.
Finally, late one night after the project was nearing deadline, I happened to set my hyper-terminal configuration incorrectly, and miraculously the meters began to communicate!
The only way I could communicate with any of the 20 of these meters on the job -- if I have the meter parity set to ODD -- is if the master's parity is set to NONE. This applies in both RS-232 broadcast mode and RS-485 mode.
Apparently, there was a minor error in the setup software in these meters that had the parities identified wrong. Perhaps the manufacturer never tested them at every different parity setting. Probably their default parity, EVEN, was correct, and ODD and NONE were switched in the setup menu. It is hard for me to believe that Newportís tech support people have not encountered this problem before and were not aware of it.
This entry was submitted by Kim Ground and edited by Rob Spiegel.
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