National Instruments NI 9411 Operating Instructions And Specifications page 10

An example of a differential device is a differential encoder.
A differential encoder has phase A, phase B, and index signals.
Use the phase A signals to measure rotational speed. Use the
phase B signals to measure direction. Use the index signals to
measure the number of rotations.
Connect each of the signal pairs of the encoder (phase A, phase B,
and index) to a pair of DI pins. Figure 3 shows the connections for
one differential encoder.
© National Instruments Corp. 9 NI 9411 Operating Instructions and Specifications
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com