Modbus RTU/TCP communication check in CodeSys

CODESYS
1

Hi everyone,

As I am busy with projects that contain many modbus RTU remote IO’s i was wondering if someone knows the best practices to do the modbus communication check in PLC program. I would like to implement a fallback to the devices if modbus fault occurs and there is no communication with with one of nodes? i would apprecieate if someone can give me some advice.

2

image
image905×441 31.3 KB

I thought about whether it might be possible to check via xError, but I haven’t tested it directly myself.

3

On RTU I would check if xError is TRUE after xTrigger is reset by the function block.

For TCP you can use ComState if you are using the configurator.

image
image499×159 3.74 KB

4

Hello

As somebody has mention before, the way to monitor if a slave is alive or not is via the properties of the instance created with the Modbus configurator.

Any slave you declare is a instance of type “ModbusSlaveComPort”

image
image1151×215 76 KB

you can fin the declaration at library “IoDrvModbus” and its interface is as shown

image
image1025×275 85 KB

Output variable “xError” will become TRUE when communication fails. if you left “Auto-restart communication” check box unchecked, in the Mosbus Master (client)

image
image1215×548 134 KB

then you can get the slave back at the poling loop by programming issuing a rise edge at “xAcknowledge” input variable of the corresponding Slave (Server), doing it in ST language may be as simple as following

image
image919×312 67.9 KB

We used this trick in a project where we may have variable number of slaves and we just want to run time just get into the pooling cycle just some of then.

Best regards.

5

Hi Guys, thank you for all your responce. Now i get the idea how can i get it done.