RTU Master RTU Slave RTU Module

• Master = com port paramters 
• Slave = group modules
• Modules = Modbus ID (UnitID) and function code

if you want to R/W to a slave you need at least 2 modules per slave.
The address in Module is used as address offset in the standard Modbus table.
Use Uint16 for channel representation.

For manual control of R/W, disable Continous and use SendOp flag.

If you let the PLC control de R/W (Process=1 and specify PLC thread), then in each scan cycle the PLC will use the module properties to send a commands to all modules it controls.
In the example case, the PLC will send one command to slave id = 1, command code = 03 (read holding registers), address = 0x0000 (default 40001).
For Continous=1 R/W is automatic.

If you need manual control of read/write cyle, which is somewhat disirable for slow connections, you need to set Continous=0, and use the SendOp flag from PLC or application.

<img title="RTU" src="/wiki/images/b/b9/RTU_M_S_M.gif" alt="RTU" width="1200" height="626" />