M172: How to change RS485 Device Node Modbus Address on the fly

[pete238479]

I have an M172P running as a Modbus RTU Master on RS485-2, connected to an RTU device, and SUCCESSFULLY reading data from the device.

Knowing the Modbus RTU Unit ID of the device, I can set it in the 'Modbus Address' field of the Modbus Device Node for the RS485-2 resource in the Resource tree, rebuild, download, and run. No problems here.

What I want to do is change the Unit ID (Modbus Address) at runtime, either on-the-fly, or at least during initialization (in the Init task) or at reboot (in the Boot task). For example: save the desired Modbus Address as an EEPROM parameter (ex. DEVICE_UNITID), then in the Init task, so something like:

         sysMbMRtuNodeStatus[0].addr_1 := DEVICE_UNITID;

This throws an compiler error, presumably because sysMbMRtuNodeStatus is R/O, or addr_1 is R/O.

         INIT(5) - error G0008: sysMbMRtuNodeStatus[0].addr_1 => Invalid access to variable

I will assume that doing the same operation in response to an HMI button would also not work

Is there any way to update the Modbus Address (Unit ID) that an Modbus RTU Device Node is using at runtime? Specifically, is there any way OTHER than doing MB RTU master requests from a program (where the modbus request function blocks could take an arbitrary Modbus Address (unit ID)) ?

Thanks!

Details:

Machine Expert HVAC 1.6.0

Target: M172P, BIOS 596.14

[M_Cervati · Accepted solution]

Hi,

It is not possible to change the ID node at run time using modbus messages in Configuration.

But it is possible to change the ID node at run time using the modbus on event mode with FBs dedicated for modbus master communication.

In attachment a modbus module to present how to use both mode of modbus communication. Modbus on event is showed starting from slide 7.

BR

Marco Cervati