Unitronic M91 PLC talk to external devices via MODBUS RTU MASTER

Hello,
I not very familiar with this protocol, any body knows or have an example of a ladder program to talk with external devices trough serial RS 485 network vith the protocol MODBUS RTU master.
I using a simple PLC M91 from Unitronic that have a serial port 485 and have built-in RTU MODBUS master protocol communication .
Any help will be very appreciate.
Thanks in advance

ericsul said:

Hello,
I not very familiar with this protocol, any body knows or have an example of a ladder program to talk with external devices trough serial RS 485 network vith the protocol MODBUS RTU master.
I using a simple PLC M91 from Unitronic that have a serial port 485 and have built-in RTU MODBUS master protocol communication .
Any help will be very appreciate.
Thanks in advance

Click to expand...

Hello Eric,

If you are familir with how the M91 handles special functions (through dedicated system registers), it's really not difficult. I've only used the M91 as a modbus slave.

There are modbus example programs for master and slave modes in the examples section of the U90 directory. In U90 Ladder, go to the Help Menu, and click on "Sample U90 Projects" to browse.

Edit:

The example programs show interaction between two M91's. To interact with a non-Unitronics device, the protocol is the same, but the address is slightly different because Unitronics addressing begins with "0" and modbus standard addressing begins with "1". The help file topic includes a section on slave addressing with examples.

If you have any specific questions, don't hesitate to ask, or contact Unitronics directly. The phone number is on the web site, or the email is [email protected]

Good Luck!

TM