Modbus RTU Master Comm from M340

I'm trying to communicate to a Modbus RTU slave device using an M340 as the Modbus master using READ_VAR and WRITE_VAR.

I'm connecting directly to the slave device from the serial port of a M340 P342020 processor over RS-232. To address the device, do I use the ADDM function block, like this:

ADDM(’0.0.0.x’)

Where x is the slave Modbus ID?

I know that Modbus TCP requires specifying a network interface. Is this not necessary with Modbus RTU?

Thanks for any help.

From Unity help it should be as follows, so it looks like you have it correct assuming you have got everything at 0 ?

Otherwise it may be your READ_VAR not configured correctly ?

For addressing a device using Modbus protocol, parameter IN takes the form:

‘r.m.c.e.MBS’

Where:

r: rack number (rack)

m: module position

c: channel number (channel) of Modbus port (0)

e: Modbus slave number (equipment) (range 1 to 247)

MBS: for addressing a Modbus server