National Instruments NI 9401 - Manual (Page 7)

© National Instruments Corp. 7 NI 9401 Operating Instructions and Specifications
Each channel has a DIO pin to which you can connect a digital
input or output device. The eight DIO channels are internally
referenced to COM, so you can use any of the nine COM lines as
a reference for the external signal.
The DIO channels are grouped in two ports, one containing
channels 0, 1, 2, and 3, and one containing channels 4, 5, 6, and 7.
You can independently configure each digital port in software for
input or output. Note that all four channels in the port must share
the same line direction. Refer to the software help for information
about configuring ports on the NI 9401.
Each channel also has a pull-down resistor and includes
overvoltage, overcurrent, and short-circuit protection. Refer to the
Specifications section for more information about input thresholds
and overvoltage protection. Refer to the Overcurrent Protection
section for more information about overcurrent protection.