Sinking VS Sourcing: Wiring Digital I/O Correctly

 Sinking and sourcing describe the direction of conventional current flow between the I/O module and the field device. They are not a property of one component alone — a sinking input must be paired with a sourcing field device, and vice versa. Get the pairing wrong and the circuit simply never completes, so the input never turns on.

The pairing rule: A sourcing I/O circuit supplies (sources) current to a sinking field device. A sinking I/O circuit is driven by a current-sourcing field device. Put simply: field devices wired to the positive (+V) side of the field supply are sourcing devices; field devices wired to the negative (DC common) side are sinking devices. A sinking input module therefore expects a sourcing sensor, and a sourcing input module expects a sinking sensor.

A practical way to remember it: Trace the current. For a sourcing output driving a load: current leaves the output terminal, flows through the load, and returns to common. For a sinking input: the sensor supplies +V to the input terminal, and the module provides the path to common. If you can name where +V originates and where common returns, you can tell which arrangement you have.

Regional Convention: Conventions differ by region. In much of Europe, DC sinking input and sourcing output module circuits are the commonly used options, so a sensor and module bought for one market may assume the opposite polarity of what a technician trained elsewhere expects. Always check the module's wiring diagram rather than assuming