Drop-out problems with PCI-85 Modbus Plus Card

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Thread Starter

Roger Valmadre

I'm using a Schneider PCI-85 Modbus Plus card in a Citect (TM) SCADA IO server to communicate to PLCs on a Modbus Plus network. It drops out all the time, and the Citect server is constantly re-starting the Citect driver for most of the devices.

(Not to be confused with the MBX driver; that stays running as far as I'm aware. MBX Demo shows similar drop-out behaviour for the nodes on the network.)

In exactly the same situation, a SA-85 card (which is an older ISA bus card) works with little trouble.

All software appears to be installed correctly.

Has anyone else had similar trouble with these PCI-85 cards?
Thanks in advance,
kind regards,
Roger.
 
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William Hinton

Roger,

Our facility uses Modbus Plus extensively, we use the ISA and PCI Modbus Plus interface cards and we have found that MOST Modbus Plus problems are caused by installation problems. Do you read 60 to 90 ohms from the blue wire to the white wire? Do you have a minimum of 10 feet of cable between nodes? Are the blue and white wires open (not grounded) with respect to ground? Did you use the proper crimping tool for connector installation?
The Square D tech support and website could help:
http://www.squared.com

I hope this helps,
William Hinton
Sr. Electrical Engineer
Delphi E&C
 
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Roger Valmadre

Thanks for your prompt reply, William.

I did a quick check on resistance between cores, and for grounding of the cores, and both measurments were OK.

However, I do know that a number of our node to node distances are less than 10ft; a few may be as close as 5ft apart. (Most of this cable was installed 10 years ago while we were very new to Modbus Plus, and the network wasn't as big.)

I know that these are a couple of tough questions, but if anybody knows the answer, I'd be most grateful:
How critical is this node to node distance?
What effect does a too close node to node distance have?

(I suspect that it would cause problems with passing the message and token from node to node, so both get "lost" and the token gets re-generated, but that's just an educated guess.)

Kind regards,
Roger Valmadre,
Electronics Tradesman,
CSR Sugar
rvalmadre@remove_this_bit.csr.com.au
 
Good Morning,

I know that this is a little late, but earlier this year I found the problem, so for the benefit of any searchers, here it is. :)

I decided to re-terminate ALL the Modbus Plus plugs in our factory (> 25 I think), checking for a performance improvement each time. There was one plug which had a stray connection from the shield to one of the twisted pair signal wires, grounding it at least partially.
(As Homer Simpson might say: "D'OH!")

I can reproduce the errors which used to flood the network very closely by re-establishing this stray earth link.

Since I performed this repair, the CRC errors (as seen by MBPStat) have dropped to almost zero.

I have yet to re-trial the PCI85 MB+ cards, but I suspect that they'll work fine now.

Kind regards to all,
Roger Valmadre
rvalmadre@_remove-this-bit_sucrogen.com
 
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