What is sinking and sourcing in PLC?



The Sinking and Sourcing are the input/output module cards used in PLCs. Conventional Relay cards are simple and very easy to the interface but suffer from the mechanical limitation of low operation and limited lifespan. These problems can almost overcome by using sinking and sourcing digital cards.

A sinking digital I/O (input/output) provides a grounded connection to the load, whereas a sourcing digital I/O provides a voltage source to the load.

Sinking input:

The switching element used in sinking card is NPN, so they are also known as NPN cards

Sinking output:

Advantage of sinking:

  • Ability to switch voltage higher than the card supply voltage

But two power supplies are required for proper interfacing

Sourcing input:

The switching element used here is PNP and so they are known as PNP cards.

Sourcing output:

Advantage of sourcing:

  • Easier interfacing since only one power supply needed.

  • The voltage load is limited to card supply voltage

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