Modbus TCP issues

cjh

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• Jul 25, 2014

• #16

When I look at the captures, it appears to me that you are not seeing the traffic between the Micrologix and the Omega controller which would be normal when using a true unmanaged switch. You will need a managed switch with port mirroring capability or a 'true' ethernet hub as Ken stated earlier. The unmanaged switch is filtering out the messages from your PC.

 

kwyshak

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• Jul 25, 2014

• #17

I managed to find a managed switch through my IT group. Here I do see a communication to 192.168.1.4 (Omega controller).

New wireshark file attached.

I do appreciate your help on this.

Thanks.

 

cjh

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• Jul 25, 2014

• #18

Based on that capture, I would try a Unit ID of 1. The Micrologix request looks good and there is a TCP ACK from the Omega controller so it looks like the Ethernet side is good, it just doesn't like the Modbus request.

It is not 'supposed' matter what the Unit ID is on Modbus TCP unless you are using a TCP to RTU bridge but a lot of devices care.
The only difference I see in the successful message from Modbus Poll versus the unsuccessful message from the MicroLogix is the Unit ID.

 

kwyshak

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• Jul 25, 2014

• #19

OK, now I do think I see something.

PLC to OMEGA looks like this:

00 06 00 03 00 27 00 01

MODBUS to OMEGA looks like this:

00 06 01 03 00 27 00 01

The unit ID is 0 and should be 1. Where is that controlled in the PLC? If I can change that I may have my answer?

 

kwyshak

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• Jul 25, 2014

• #20

Progress. Now I DO get a response, but it errors out immediately.

Error code 82. Illegal address.

Wireshark shows this is being sent...

06 01 03 00 27 00 64

 

kwyshak

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• Jul 25, 2014

• #21

Got it!!

Thanks all for your help on this!

 

cjh

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• Jul 25, 2014

• #22

So now you are trying to request 100 registers. Will that device support that?

 

Hojland

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• Jan 23, 2018

• #23

Hey Kwyshak.

I have the same problem. How did you sloved it?

 

makr8100

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• Feb 5, 2018

• #24

I suggest QModBus, similar features as Wireshark as it pertains to Modbus but nicer GUI, but not as universal (i.e.- only works on Modbus).
http://qmodbus.sourceforge.net/

There's a mobile version written by yours truly, but I haven't maintained it lately and it won't run on current versions of Android. It worked well on Android 2.x, and iirc it ran on early 4.x versions.
https://sourceforge.net/projects/mqmodbus/