National Instruments NI 9425 - Manual (Page 8)

NI 9425 Operating Instructions and Specifications 8 ni.com
Each channel has a DI pin to which you can connect a digital input
signal. The NI 9425 has four common pins, COM, that are
internally connected to the isolated reference of the module.
The NI 9425 has sinking inputs, meaning that when the device
drives a current or applies a voltage to the DI pin, the pin provides
a path to COM for the current or voltage. The NI 9425 internally
limits current signals connected to DI. For more information about
input current levels, refer to the
Specifications section.
You can connect 2-, 3-, and 4-wire sourcing-output devices to
the NI 9425. A sourcing-output device drives current or applies
voltage to the DI pin. An example of a sourcing-output device is an
open collector PNP.
Connect the sourcing-output device to the DI pin on the NI 9425.
Connect the common of the external device to the COM pin. Refer
to Figure 2 for an illustration of connecting a device to the
NI 9425.