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RS485 MODBUS - optical network

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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.

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  • MVP 2023

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.

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