Power-io I/O Modules example:

Terminal 4 on the I/O module is typically wired to a PLC's input card, or other product, that is expected to see a "sinking" signal, or pull-down signal.

When terminal 4 is at zero, the circuit is completed, and the external device turns ON.

The Power-io I/O modules can sink up to 100mA.

IO Sinking Circuit