PNP or NPN sourcing or sinking?
Sourcing and sinking are not used consistently across manufacturers. My understanding is a sinking devices supplies common(NPN) and a sourcing device (PNP) supplies power. A sinking input requires a sourcing sensor and a sourcing output requires a sinking load. Pneumatic valves are often called sourcing but require a sourcing output (one that supplies power).
With that all said, in the US sinking inputs and sourcing outputs. I feel this is the best way because if a wire breaks and shorts to ground you cannot get a false input. If the output wire breaks and shorts to ground the load device will not be actuated. I have seen many times wires getting pinched and shorted to ground.
In Japan tends to be sourcing input and sinking output. Europe I believe is sinking inputs and sourcing outputs.
when talking about the IO cards, sourcing means they send out 24V and sinking means they are expecting 24V in. Sensors are generally not referred to as sinking or sourcing and instead referred to as PNP or NPN. PNP = Sinking inputs (you put in 24V to turn an input on) and Sourcing outputs (24V comes out of them when you turn them on). NPN = Sourcing inputs (24V comes out of the input and connecting that 24V to common turns the input on) and Sinking outputs (when turned on, the output connects to common). Generally, you always use PNP, Sinking inputs, sourcing outputs. You may find many different type of Remote I/O modules based on different protocols ( like Modbus, Ethernet, Profibus, CAN bus, etc) and different I/O type Sink/Source Wet/Dry etc at below link: Remote I/O Data Acquisition | ICP DAS USA Inc - Data Acquisition And also see below link which is very useful to select what you want: http://www.icpdas-usa.com/m7000_analog_digital.php#digital?r=mohsen