Modbus in MCC

jedft · Aug 1, 2007

Hey all, I have a guy in the field trying to T/S a problem with a MCC. There are something like 24 or so buckets with Motor Logix Plus II modules on 2 seperate Modbus networks. These are linked together on 2 seperate Modbus to Ethernet converters then via Ethernet going to the PLC. Problem is, when any one bucket on one of the Modbus networks is powered down, the PLC (Momentum processor) loses comms to that entire Modbus network. The detials are kind of sketchy because I'm not there... I guess I'm looking for a "DUH" setting or something here

Thanks!

tragically1969 · Aug 1, 2007

Is it a new system or one that has been working fine then stopped working ?

Are the modbus converters these ?:

http://www.powerlogic.com/intl/products.cfm?id=17

We had issues with the timeout settings on these units, you need to make sure they are set lower than that of your PLC comms timeout otherwise they sit there until they time out and move on to polling the next device

dogleg43 · Aug 1, 2007

***************************************
I guess I'm looking for a "DUH" setting
***************************************

What a great phrase!! Can I borrow it?

jedft · Aug 1, 2007

tragically1969 said:
Is it a new system or one that has been working fine then stopped working ?

Are the modbus converters these ?:

http://www.powerlogic.com/intl/products.cfm?id=17

This is a system that is currently being commissioned. I'm pretty sure that they are the EGX400 modules. I know he's played with the timeouts on the I/O scanner, but I don't think he's done anything with the converters themselves.

jedft · Aug 1, 2007

dogleg43 said:
***************************************
I guess I'm looking for a "DUH" setting
***************************************

What a great phrase!! Can I borrow it?

No, but it's for sale!! :nodi: J/K.. sure you can!

allscott · Aug 1, 2007

The duh setting parameter is the one that is found shortly before a machine starts working, the DOH! setting is the one found shortly before a machine stops.

tragically1969 · Aug 1, 2007

jedft said:
This is a system that is currently being commissioned. I'm pretty sure that they are the EGX400 modules. I know he's played with the timeouts on the I/O scanner, but I don't think he's done anything with the converters themselves.

I have never used the EGX400, they will be similar, they do a little bit of buffering and control so it may be these, firstly can you read registers using the diagnostics tool in the EGX when one of the other devices is off ?

Its hard to say but i presume you are using the READ_VAR command to read registers from the EGX over TCP, you need to make sure that the READ_VAR's are doing some sort of error checking that the responses are correct

If you have something running the request for each read make sure each read has a reset if it times out or recieves an error from the modbus device

Also back the timeout right off on the EGX to a second or something

jedft · Aug 1, 2007

tragically1969 said:
I have never used the EGX400, they will be similar, they do a little bit of buffering and control so it may be these, firstly can you read registers using the diagnostics tool in the EGX when one of the other devices is off ?

Its hard to say but i presume you are using the READ_VAR command to read registers from the EGX over TCP, you need to make sure that the READ_VAR's are doing some sort of error checking that the responses are correct

If you have something running the request for each read make sure each read has a reset if it times out or recieves an error from the modbus device

Also back the timeout right off on the EGX to a second or something

We are using the Ethernet I/O Scanner (Concept software) to read/write to the Modbus modules through the converters. There is a Health Timeout and a "Rep Rate" Which I think he has set at 1500ms and 500ms respectively. I'm going to tell him to look into the diag. tool for the EGX. I don't think he's used that at all yet. This MCC was originally set up by the factory and now we are trying to make it work. Thanks!

kittydog42 · Aug 1, 2007

The way that I handle this problem is actually really easy. I set a relay output in the remote PLC to be always closed. I hook that output up to my main PLC as an input I call "remote link". I only allow the READ_VAR command for that remote PLC to take place if the remote link shows that the PLC is actually powered up. Now, this does not take care of every potential situation, but it does keep the network moving fast and it makes the whole process more simple.

Modbus in MCC

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