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.

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