While connecting field devices to Programmable Logic Controllers we always get confused about sinking and sourcing operations. Also this sinking & sourcing concept is useful to answer PLC interview questions . Sinking and sourcing operations refer to electrical configurations of the circuit in the module & field input devices. When a device provides current when it is ON it is said to be sourcing current . When a device receives current when it is ON it is said to be sinking current. There are both sinking & sourcing field devices as well as sinking & sourcing input module. But it’s a common practice to use input module in sinking current mode. · Field devices on the positive side (+VDC) of the field power supply are sourcing field devices. · Field devices on the negative side (DC COM) of the field power supply are sinking field devices. · ...