PLC Communication Issues

[AlexO35]

I am a former military electrician gone manufacturing. I'm looking for a little help with PLC serial networks. We are using IDEC PLCs in a daisy chain serial network linked to a main control PC running KEPServerEx. My issue is that as the day goes on, the PLCs "fall off" the network. KEPServer gives me a "channel not responding" alarm and that PLC goes off line, the others drop off one by one over time. Reseting the PC brings the network back up, but there has got to be a better way. Thanks for any help you can give.
--Alex

 

[TWControls]

What type of serial network are we talking about here?

What's the distance?

What cabling are you using?

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TW Controls -

http://www.twcontrols.com

 

[eeinpa]

If, by chance, you are using a multi-drop RS485 network, be warned that some funny things can happen if nothing is left driving the bus... In a previous job we had a lot of trouble with customers using this network without adding some kind of biasing resistors to pull signals to a known valid state when no drivers when enabled. Some RS485 interfaces (e.g. PC cards) have jumper-enabled resistors for this purpose.

 

[AlexO35]

I am not sure about the type of network. I'm running the Idec FA3S series PLCs with an Idec FA-1Junior controling the serial network. Distance is roughly 100yrds to the furthest unit in the chain. Cable is a pair of four wire shielded, one for TX the other for RX. Thanks for the help.
--Alex

 

[pooslinger]

I don't deal with Idec very often mostly Allen Bradley but some of the small networks I have at my plant are running DH-485. One bad converter or one bad processor can bring down the whole network randomly with out warning. Hope you figure it out, sounds like a pain.

 

[eeinpa]

I am not familiar with this hardware, but it sure sounds like multidrop RS485, considering the distance and wiring you describe.

I suggest you "google" these words:
rs485 four wire termination biasing

You will find several good application and troubleshooting notes on the topic. The biasing is where I would look first, as I don't think your distances are enough to worry much about termination.

I must warn you, I could be pointing you in the wrong direction here, as I do not have particular experience with your hardware. But it is certainly something to look at!

If you have a scope, you may be able to view the lines and see what they are doing. The curse is that the added capacitance will sometimes "fix" the problem. Until you quit looking! Grrr. Thanks, Mr Heisenberg

Good luck!

 

[AlexO35]

Thanks for the help. I'm jumping into someone else's shoes here and picking up the problems that came along with the job. I'll keep poking around and see whats what. Thanks again.
--Alex

 

[danw2]

Somewhat similar symptoms for a DH+ network on a thread at PLCs.net . . .

http://www.plctalk.net/qanda/showthread.php?t=28374

 

[Macbeth]

Look at the PC port settings. Increase REPLY TIME OUT and max number of retries.

Also in the KEPServer increase the Pulling time (Look under the topic).

Iam not sure how much data you are pulling and at what speeds but I often find people over taxing thir systems.

 

[AlexO35]

What are the limiting factors for data/speed on these systems? I currently have a KEPWare rep helping me out on the specifics, but am curious as I haven't delt with this for long.
--Alex

 

[djs]

Hello;
I would check into the bus termination of the network. If left unterminated, wierd things can happen.