Relay Output Module Vs. Sourcing Digital Output

Solution

A sourcing digital output module:

Uses three pins to interface with a device: supply voltage, digital output, and common voltage.
External power is connected between the supply voltage pin and the common voltage pin with the ground of the power supply being connected to the common voltage pin.
The device (or load) is wired between the digital output pin and the common voltage pin with the ground of the device (or load) being connected to the common voltage pin.
When the channel is on, the device drives current from the supply voltage pin to the digital output pin, thus delivering power to the device (or load).
All the channels may share a supply voltage and a common voltage or each channel may have their own supply voltage and channel voltage.

A relay output module:

Uses two pins to interface with a device: CHa and CHb. CHb will sometimes be a common return line name COMx.
External power and the device (or load) are connected such that, when the relay is closed, the configuration of the circuit is such that the device (or load) and external power are connected in parallel or in series (Note: The circuit configuration will depend on what is expected by the device or load).
- To connect the external power supply and load such that they are in parallel when the relay is closed:
  - Connect the positive end of the external power supply to CHa and the positive end of the device (or load) to CHb, or vice versa.
  - Connect the negative side of the external power supply to the negative side of the device (or load).
- To connect the external power supply and load such that they are in series:
  - Connect the positive end of the external power supply to CHa and the negative end of the device (or load) to CHb or vice versa.
  - Connect the negative side of the external power supply to the positive side of the device (or load).
Each channel requires an external power supply.