Sourcing and Sinking Concept in PLC

  
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.

·        Sourcing field devices must be connected to sinking I/O cards and vice versa.

·        Sinking field devices must be connected to sourcing I/O cards and vice versa.