Modbus is awesome

[HJTRBO]

Had a good break down today. I had random drive alarms on the scada page causing the clients plant to shut down. After following through the code i determined that the fault register reads from certain drives were not being received. Unfortunately it would only happen once every 10 to 25 minutes. The original programmer was writing FFFF to the respective fault destination registers in the plc and allowing up to 10 seconds for the receive read to update the register with 0000 which if it wasn't done would set one of the fault bits.
After following a calculated hunch I found a damaged cable close to where I thought the problem was and repaired it. Sweet, all working now.

Now for the indestructable part, this plant has all the modbus 485 done in cat5 obviously unshieled, running hard up against mains cabling (cable tied in the same loom) and unshielded VSD cables (yes, unshielded). Probably 600m all up with 15 slaves. I cant believe this network works. I read alot about how important shielding and grounding is of field bus networks, but this place throws all that out the window. Its hard to describe in words what I saw but its a miracle this place runs. And at 19.2 baud.

Its not good to know what you can get away with!

 

[danw]

CAT5 is twisted pair. Twisted pair makes a huge difference in noise reduction; so much better than THHN.

Are the end points terminated with resistors?

 

[HJTRBO]

Yes.

 

[rootboy]

I agree, Modbus rocks! We use twisted-pair cable DeviceNet with properly terminated resistor-capacitor network and it still gives us no end of troubles (so much for twisted pair).

We are now getting bus off conditions on a different network on our Prime oven. At least it it's a different network this time...

On the other hand, our GE Genius rarely gives us trouble even though its networks are much longer.

It isn't that its Modbus per se, but that you are running a current loop rather than voltage that's making all of the difference.

And it doesn't hurt that you are running at only 19.2K. But the fact that you are doing it all in CAT5 is pretty impressive.

 

[BobB]

I normally use twisted pair screened with the screen grounded at only 1 end of course. The cable is quite inexpensive and is SCREENED importantly. I use the same cable for all analogue signals as well. It is 7/0.20 communications cable. Have not had a problem yet.
If the run is long and the resistance is increased, a larger core cable is used as sometimes it becomes almost impossible to 'see' the end terminator and comms errors then occur. That is something that is often missed when installing comms lines.

 

[danw]

It isn't that its Modbus per se, but that you are running a current loop rather than voltage that's making all of the difference.

RS-485 is not a current loop. Current loop converters are still commercially available, but in my experience, are rarely used.

RS-485 uses balanced line, differential voltage driver/sensing:

Dan

 

[rootboy]

RS-485 is not a current loop. Current loop converters are still commercially available, but in my experience, are rarely used.

Yup, my bad. I don't know what I was thinkin' (drinking?)