Instruction Manual

Rockwell Automation Publication 1769-UM006E-EN-P - July 2013 25
Module Operation Chapter 2
X1 Quadrature Encoder Mode Selection
In this mode, when a quadrature encoder is attached to inputs A and B, the count
direction is determined by the phase relation of inputs A and B. If A leads B, the
counter increments. If B leads A, the counter decrements. In other words, when B
is low, the count increments on the rising edge of input A and decrements on the
falling edge of input A. If B is high, all rising transitions on input A are ignored.
The counter changes value only on one edge of input A as shown in Figure 5.
For more information see
Direction Inhibit and Direction Invert Output
Control Bits on page 21 and their effect on Quadrature signals on page 27.
Table5-UpandDown Counting
Direction
Inhibit Bit
Direction
Invert Bit
Input A (count) Input B (direction) Change in
Count Value
00 0, 1,  1
0, 1,  -1
 0
01 0, 1,  -1
0, 1,  1
 0
10 0, 1,  1
0, 1,  1
 0
11 0, 1,  -1
0, 1,  -1
 0
TIP
When both A and B transition at the same time, instead of in the defined
90° phase separation, the quadrature signal is invalid.