Which statement correctly describes the characteristics of Sink Input and Source Input?

Which statement correctly describes the characteristics of Sink Input and Source Input?

• A. Sink Input is Active High; the switch is on the positive side; Voltage flows from PLC to device; If device is enabled voltage will be present at PLC terminal.
• B. Source Input is Active High; The switch/input/device is on the positive side of the circuit or source voltage. Voltage flows from PLC to device.
• C. Sink Input is Active Low; the switch is on the ground side; Voltage flows from power source through switch/input/device to PLC. If device is enabled voltage will be present at PLC terminal.
• D. Source Input is Active Low; the switch/input/device is on the negative side of the circuit or source voltage.

Answer: (C)

Sink inputs are designed to be activated by pulling the input channel toward ground, so the switch or load sits on the low side of the circuit. When the external device provides power and the switch on the ground side closes, current flows into the PLC input and to ground. The PLC senses the input as ON when the line at the input is pulled toward 0 V (low), hence the term active-low. This is why the description that the switch is on the ground side and the input is active when it’s pulled low correctly reflects sink input behavior. In contrast, a source input would be active-high, with the switch on the positive side so the PLC input sees a high signal when the external device is energized.

Sink inputs are designed to be activated by pulling the input channel toward ground, so the switch or load sits on the low side of the circuit. When the external device provides power and the switch on the ground side closes, current flows into the PLC input and to ground. The PLC senses the input as ON when the line at the input is pulled toward 0 V (low), hence the term active-low. This is why the description that the switch is on the ground side and the input is active when it’s pulled low correctly reflects sink input behavior. In contrast, a source input would be active-high, with the switch on the positive side so the PLC input sees a high signal when the external device is energized.