National Instruments NI 9426 specifications - page 7

© National Instruments Corp. 7 NI 9426 Operating Instructions and Specifications
Each channel has a DI pin to which you can connect a digital input
signal. The NI 9426 has four supply pins, Vsup, that are internally
connected to the isolated reference of the module.
The NI 9426 has sourcing inputs, meaning the DI pin sources
current from Vsup to the sinking output device. The NI 9426
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 sinking-output devices to
theNI 9426. A sinking-output device provides a path from the
DIpin to a voltage below Vsup. An example of a sinking-output
device is an open collector NPN.
Connect the sinking-output device to the DI pin of the NI 9426.
Connect the positive power supply lead of the external device to the
Vsup pin. Refer to Figure2 for an illustration of connecting a device
to the NI9426.