pi14 Posted July 11, 2011 Report Share Posted July 11, 2011 Hello. I want to communicate between two M91 plc-s (M91-2-T38) with RS485, MODBUS protocol. When i connect the PLC-s with the cable the communication works well. My problem is, that the distance between both PLC-s is to long, so i must use optical fibers. I have two RS485<->optic converters. But the communication dont work. So if you have some ideas what can be wrong or what i must to observe with that kind of connection, please let me know. Is there any other possibility to communicate between those two PLC-s (some signals (MB, MI)). Thank you. Quote Link to comment Share on other sites More sharing options...
MVP 2021 Joe Tauser Posted July 12, 2011 MVP 2021 Report Share Posted July 12, 2011 This is a question for your converter supplier, and it may or may not matter whether you are using termination resistors. I've actually experienced the RS485 port on a Unitronics over-driving a converter and I had to use a terminator to knock the voltage level down on a very short (2m) cable. For some reason it didn't work at 10 Vpp, but it works great at 6 Vpp. You need an oscilloscope to see this. Before anybody at the Home Office chimes in, no, it wasn't the Unitronics RS232 to RS485 converter. I had to go to USB and used "another" brand. What kind of converters are you using? Joe T. Quote Link to comment Share on other sites More sharing options...
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