Communicating with a Microcontroller using MODBUS

Hi All,
I plan to set up a communication between a Microcontroller system and an UNITRONICS JAZZ PLC, using MODBUS. See sketch attached.

I will be reading only one byte of information from the Microcontroller at any instant. The value could be anything from 0 to 100. Can someone tell me as to what will be the sequence of bytes that I need to send to the PLC from the Microcontroller?

Thanks in advance.

Ron

The MODBUS specification available here is very clear.