Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 13)

1–5 Overview of the Very High Speed Counter Module
Figure 1.2
Block Diagram of Encoder Mode
From Encoder/Pulse Generator
Phase A
Terminal
Phase B
Terminal
(direction sense)
Gate/Reset
Terminal
1771ĆVHSC
10678ĆI
Direction of Count
The module can count either up or down, depending upon the
condition of the B input for each counter. In encoder applications,
the counter will increment on the leading edge of Phase A, while
phase B determines the direction of the count.
You also have the option of X1 and X4 multiplying of the input
pulses. 1.3 shows the relationships between phases A and B for
forward and reverse directions in encoder applications.