Silly dart at the board question about Modbus

[SiriusMark]

I read an article that said Modbus *may* not require it for runs that are 100 feet or less. Do you think that is bogus? I am going to double check around the field I work at, but I'm about 95% certain they aren't using resistors. I can promise you this: I'm going to start using them regardless of what the last guy did. That way I don't have to play that stupid game when connectivity issues arise.

 

[danw]

This could mean that it is set up wrong or wired backwards.

One of the first things I do when I am in your shoes is swap the wires of the problem device. Sometimes you just ignore what the book says and try swapping the Tx and Rx wiring. There are only a few ways you can wire the thing and I have never blown anything up by having it wrong. Someone correct me if I am wrong but I don't think it can hurt anything and is often simpler and faster than interpreting a manual.

You are very much 'on-track'. The EIA standard does not define the terminology used for the A/B or (+)/(-) or D+/D- wires in a 2 wire RS-485 network. Some manufacturers label them one way, some another.

Wiring should be (+) to (+) to (+) and (-) to (-) to (-) or A to A to A, but sometimes the labeling is backwards and it becomes A to B to A.

Connecting the wire backwards will not damage the line drivers, but it will NEVER work with the wires backwards.

Swapping lines is one of the first things to try.

 

[danw]

I read an article that said Modbus *may* not require it for runs that are 100 feet or less. Do you think that is bogus?

No, it isn't bogus. The slower the baud rate and the shorter the cable, the less termination is required.

According to Lantronix (a Digi competitor), one can calculate whether termination is needed (attached article)

This Youtube video has an oscilloscope showing the reflected energy that the termination is intended to dissipate
https://www.youtube.com/watch?v=zrDxSM91Jcg

 

[SiriusMark]

You are very much 'on-track'. The EIA standard does not define the terminology used for the A/B or (+)/(-) or D+/D- wires in a 2 wire RS-485 network. Some manufacturers label them one way, some another.

Wiring should be (+) to (+) to (+) and (-) to (-) to (-) or A to A to A, but sometimes the labeling is backwards and it becomes A to B to A.

Connecting the wire backwards will not damage the line drivers, but it will NEVER work with the wires backwards.

Swapping lines is one of the first things to try.

Things like this is why I like beer.

 

[SiriusMark]

Seriously though, I didn't finish the project today because of some more pressing issues, but now that I've gotten the comms stabilized, I intend to figure out the second issue tomorrow. Thank you for that article, it's something to put in my bag o' tools when I'm dealing with Modbus.

 

[BobB]

I set Modbus RTU comms up inside a control panel this afternoon from an Omron CJ2M PLC to a fuel tank monitor and a generator controller - took all of 7-8 minutes to have it steaming along. No terminating resistor either - the RS 485 run is about 10 feet.