“Sinking” and “Sourcing” terms are very important in connecting a PLC
correctly with external environment. These terms are applied only for DC
The most brief definition of these two concepts would be:
SINKING = Common GND line (-)
SOURCING = Common VCC line (+)
Most commonly used DC module options in PLCs are:
*Sinking input and
*Sourcing output module
- Sinking I/O circuits on the I/O modules receive (sink) current from sourcing field devices. Sinking output modules used for interfacing with electronic equipment.
- Sourcing I/O: Sourcing output modules used for interfacing with solenoids.
PLC AC I/O circuits accommodate either sinking or sourcing field devices. Solid-state DC I/O circuits require that they used in a specific sinking or sourcing circuit depending on the internal circuitry.
PLC contact (relay) output circuits AC or DC accommodate either sinking or sourcing field devices.