RS485 Challenge


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I connected seven devices to Sixnet RTU (VT-IPM2M-213D) using RS485 RTU protocol. I observed that when the seven is powered on, one of the devices does not work when it is in the RS485 network with the other six devices. but it works when it is the only device on the network. What can I do to resolve this challenge?
You don't say how "does not work" manifests itself - Master times out? Exception code? Whole network comes down when you add #7? What exactly happens when you add #7? Error log messages?

But, if #7 works OK stand-alone then you know how to wire it and poll it.

Did you leave the terminating resistor on #6 and then add #7?

Did you change the slave node ID value?

Are slaves 1 to 6 in the next room and #7 is 1 km away?

Does #7 have a separate signal ground terminal?
I connect 6 FT3 Fox Thermal Flowmeters and 1 echart lite flow computer having embedded multivariable transmitter. The challenge is with echart flow computer, it works at times and goes off (timeout error). Also it doesn't block the rs485 network. The challenge is not with the ID of the slaves and master because it is properly done.

i have the only one terminating resistor at the master's end (sixnet vt-ipm2m-213 RTU).
Sixnet recommends one terminating resistor at each end of the RS485 network and one set of Bias Resistors on the network. Also, I believe that with RS485, the communications baud rate is limited by the network distance. Is it possible for you to lower the data rate?
I have seen this problem caused by software the supports a fixed number of devices, maybe in this case 6, which seems a little low, or the RTU does not support the address used for the new device. Is the new device the last of a long string of these? You may be on the edge of the driver's capability. If you have the capability to monitor the message traffic between the RTU and the devices that can be a great help.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
SIS-TECH Solutions, LP

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.
Can you extend the timeout period in the master? It should not slow the throughput/replies from the other devices, but might allow enough time for the echart to reply. Maybe echart is overwhelmed with calculations and can't get around to servicing the com port.

>The challenge is with echart flow computer, it works at times
>and goes off (timeout error). Also it doesn't block the rs485 network.
RS-485 is not a protocol. Do you mean Modbus RTU? It may be that you are working with devices with different protocols or the configuration is off. If it is Modbus RTU, you can use our Modbus Master tool from a PC and connect to each device and verify communication. This will help verify your wiring and configuration are correct. If you can connect to all of the RTU individually, you can try to connect with your software. If it fails, it may be a software limitation.