Programming a Modbus master inside a PLC (Siemens TIA, Wago CoDeSys, Schneider Unity Pro) is done through standardised Function Blocks (e.g. MB_CLIENT, MODBUS_CLIENT) that encapsulate requests (Read Holding, Write Single Register, etc.) with automatic handling of retries and timeouts.

Every major PLC platform ships a vendor library of Modbus master Function Blocks: Siemens TIA exposes MB_CLIENT for Modbus TCP, Wago CoDeSys provides the MODBUS_CLIENT library, Schneider Unity Pro uses READ_VAR / WRITE_VAR, and ABB CoDeSys offers MB_RTU_CLIENT. Typical input parameters include Connect_ID, IP, Port, Slave_ID, Function code, Start_Address, Length and Data_Buffer. The Function Block transparently manages the connection, the timeout (typically 1-5 s) and retries, which makes programming dramatically simpler than implementing the wire protocol by hand.

Common trap: forgetting to wire the Done/Busy bit as a trigger condition for the next request, which saturates the block and causes cascading timeouts.