The CJ and CS series PLCs have SCU and SCB - Serial Communications Units and (daughter) Board options which are highly capable units. These units can execute Protocol Macros, which are user defined communications sequences written and loaded by the CX-Protocol program. 

The Modbus RTU protocol was a very popular protocol for use with these units. the Macro is loaded into the SCU unit, and the PMCR instruction is used in the PLC code to call the communications sequence in the unit. The unit then handled all the communications and returned the data to the user program memory location. 

To perform Modbus RTU master functions, the Protocol Macro must be loaded into the SCU or SCB. This requires the actual Macro program and CX-Protocol (part of the CX One suite of programs). 

Both the documentation and the protocol are available here

Modbus PM V5.zip