Hello All! I am new to this forum but have been doing a lot of reading and there are some very smart people posting here. I figured I would throw this one out there to see what comes up.
I have been in the industrial controls and automation industry for 20+ years and works with many different Modbus devices over the years. I recently got a YSI IQ SensorNet 284 controller for a client with the intent of communicating to it via Modbus RTU with a Productivity 1000 PLC. We have done Modbus RTU successfully to many different devices in the past with this PLC.
We setup the 284 on the bench to do some initial testing and establish communications. Initial tests were being done from a laptop running ModScan, Modbus Poll, KepServer, etc. We tried several different USB to 485 converters, ones we use in the field all the time, but could not establish communication. We tried connecting the 284 to the PLC and could not get Modbus to work their either. We also tried some older 232 to 485 converters and they would not work either. We then tried an old moldy Microflex USB to 485 converter and it worked great! To add to the mystery, the Microflex converter would not work with other Modbus RTU devices that we have in the shop - thermostats, meters, sensors, etc. Probably why it was sitting on a shelf collecting dust!
Why is this? Why does Modbus RTU communication this YSI 284 controller only work when it is connected through the Microflex USB to RS485 converter? What I know about the Microflex converter is that it does not have a 120 ohm termination resistor, it does have the A and B lines biased with 1.2k resistors, and the actual 485 chip is a MAX 13487. Some of the other converters we have can have these features (bias, terminator) turned on and off but it does not seem to make a difference.
Any ideas would be much appreciated!
- Reuben
I have been in the industrial controls and automation industry for 20+ years and works with many different Modbus devices over the years. I recently got a YSI IQ SensorNet 284 controller for a client with the intent of communicating to it via Modbus RTU with a Productivity 1000 PLC. We have done Modbus RTU successfully to many different devices in the past with this PLC.
We setup the 284 on the bench to do some initial testing and establish communications. Initial tests were being done from a laptop running ModScan, Modbus Poll, KepServer, etc. We tried several different USB to 485 converters, ones we use in the field all the time, but could not establish communication. We tried connecting the 284 to the PLC and could not get Modbus to work their either. We also tried some older 232 to 485 converters and they would not work either. We then tried an old moldy Microflex USB to 485 converter and it worked great! To add to the mystery, the Microflex converter would not work with other Modbus RTU devices that we have in the shop - thermostats, meters, sensors, etc. Probably why it was sitting on a shelf collecting dust!
Why is this? Why does Modbus RTU communication this YSI 284 controller only work when it is connected through the Microflex USB to RS485 converter? What I know about the Microflex converter is that it does not have a 120 ohm termination resistor, it does have the A and B lines biased with 1.2k resistors, and the actual 485 chip is a MAX 13487. Some of the other converters we have can have these features (bias, terminator) turned on and off but it does not seem to make a difference.
Any ideas would be much appreciated!
- Reuben