Modbus configuration

[sgull]

Hi Folks

I have a program installed on a V1040 on a site that has an OMRON PLC connected to a SCADA system.

The customer wants to take some information back to the scada via the Omrom PLC.

The Omron programmer is very protective of his set up so I cannot see what he has done.

He claims to have set up his device properly to read the information through MODBUS using a 485 wiring setup.

The communication is not working and I would like to confirm the following.

 

On the V1040 I need to set up a COM INIT block to setup the port to match the settings on the Omron PLC triggered by a power up bit.

On the V1040 I need to set up a MODBUS Config block to give the modbus ID again triggered by a power up bit.

On the V1040 I need to set up a SCAN EX block which is on when not in power up.

 

I have wired with 2 wire 485 with termination at both ends.

 

Is this all I need to do to make the information available to the PLC.

 

Regards

 

Denis

 

[Alexander]

If you are acting as a Modbus slave than you are correct. It will need the COM Init set for RS485, and Modbus configuration set on power up and you will need the Scan EX function block running on all scans other than on power up.

 

You will also need to make sure that it is terminated on the ends of the RS485 network. Besides the termination jumpers did you set the jumpers for RS485 communication?

 

Is this customer unable to read any information or is it coming in as unexpected values? It could be how he is addressing the V1040 unit that is causing issues which is reliant on the Modbus master program. Are there any status messages on the master that could help determine the cause of the problems?

[sgull]

Hi Alexander

 

Thanks for your prompt reply.

On the Omron side he says he is getting no information.

On the V1040 info mode both the Tx and Rx are showing 0 (at one stage the Rx read 4 but then stopped)

The problem is that the PLC programmer is gone off site saying that his end is correct and the problem is with the V1040.

I would believe that the slave device is much simpler than the master device.

 

Can you explain one thing.

I am using CANBUS for the I/O modules.

For this I have set a PLC NET ID block and set the NET ID at 1

In the MODBUS configuration I have set the MODBUS CONFIG NETWORK ID at 2

Am I correct in saying that he must use the modbus and not the PLC net ID.

 

Regards

Denis

[Alexander]

Denis,

 

You are correct the Net ID number and the Modbus Configuration number are two separate values. The Net ID number is for connecting to a node for programming (RS485 or Serial-CANbus connection) The Modbus configuration number is used for only Modbus communications. I would ensure that the master device is also not using ID number 2 for it's configuration.

 

It requires very little programming in order to configure a Modbus slave. Simply, the Modbus configuration need to be run on power up and the scan block needs to be continuously running. That is all that is necessary.

 

Most communication issues lie with the Modbus master program or with the physical cable connecting the devices. 

[sgull]

Thanks again Alexander

 

I have a spare V1040 that I will set up as a modbus master and use the same cable to connect between them as I am using on the Omron.

If this works it should rule out the cable and slave configuration.

 

Thanks for your help.

 

Denis

[Joe_ValleyMation]

What addresses will they be reading from your V1040? MIs and MBs only?

Regards!

[sgull]

Hi Joe

 

The customer will be reading MI values only.

Alexander's comment on the wiring prompted me to check the connections again.

I had told the electrical contractor to use the two outside pins on the RJ45 connector.

Unfortunately he used a plug with only 4 terminals, so I was using pins 2 and 5 instead of pins 1 and 6.

Apologies I should have checked before posting.

 

Regards

 

Denis

[Alexander]

Denis,

 

I'm glad to hear that you found the cause of the issue, it's certainly an easy thing to miss. I wouldn't hesitate to ask about any questions, especially related to communications as the smallest thing can cause the largest problems. Plus, it's always nice when the fix doesn't involve digging through multiple rungs of logic.