There are different types of plcs: 1. input module used for connecting digital or analogue field inputs to PLC which are transmitters or industrial switch etc. 2. same way output module used for connecting field outputs from PLC which area relays, lights, linear control valves etc. 3. communication modules used for exchange data between PLC to SCADA, HMI or another PLC. 4. Expansion modules used for expanding Input or output modules