I have a 2-wire RS485 with 15 slave devices using Modbus, this network is connected to a Fibre Optic to Copper converter which then sends the data across a fibre network to another converter which converts it back to copper, which is then connect to my PLC. The fibre network is used because of the distance and for isolation purposes.
How do I terminate this network correctly, I have had a couple of different suggestions, these are:
1) Terminate at either end of the entire network, therefore only two terminating resistors.
2) Terminate only the Slave Device end of the network.
3) Terminate each RS485 network therefore having a total of 4 terminating resistors as there is two seperate RS485 networks in opperation.

What is the correct method, or is it a case of NONE OF THE ABOVE?

 

The "correct" answer is 3. The resistor is there for reasons related to the properties of the transmission medium (copper). The electrical system on one side of the F/O can't see the other side electrically, only logically, therefore two resistors on each copper system.

In practice, I'll bet you have the FOX's right next to the PLC's with a foot or two of copper connecting them, right? On very short networks, my experience has been that the resistors sometimes do more harm than good, although I don't know why. So play around with the resistor count till you find what gives the most reliable comms.

 

Number 3 is the correct method. A copper RS485 network should always be terminated at both ends of the copper segment. Be cautious as some fiber converters have the option to have the terminator selected internal to the device and you do not want to add an additional terminator in that case. To be sure ask the device manufacturer if a terminating resistor is built in.

 

The simple answer: Both ends of both RS485 copper (twisted pair) networks require termination.

Having said that, it is quite possible that the copper/fiber converters have termination built in. Check the manuals, I've seen some that use a DIP switch or jumper to enable/disable the termination.

I've found a link I use often which describes why the termination is required and what to expect if it isn't done.

http://dbserv.maxim-ic.com/appnotes.cfm/appnote_number/763

 

Low quality transmission lines with unmatched connectors require a certain line length to exhibit their characteristic impedance. Or to
really look like a transmission line for that matter. That's why.

Regards

cww