FP-QUAD-510 6 www.ni.com
Position Counter Operation
Each position counter has a 32-bit binary counter that represents
the current position of the shaft connected to the axis. Channels 0
through 3 contain the lowest 16 bits of these counters for axes 0
through 3 respectively. Channels 4 through 7 contain the
uppermost 16 bits for axes 0 through 3 respectively.
Primarily, the position counter channels increment or decrement in
response to edges encountered at the Phase A and Phase B inputs.
The following sections describe the operation of the counter
channels.

Position Counter Input Circuit

The circuit for the position counter inputs consists of a comparator
with current limited inputs to convert from a differential to a
single-ended signal. The output of the comparator is used to drive
an optoisolator. When you apply a voltage above the threshold
voltage to the input terminal, the comparator turns on the
optoisolator, registering in the ON state.
The inputs are designed to be compatible with single-ended TTL
encoders or differential encoders supplying a differential voltage of
at least ±0.3 V.
Each encoder input has a lowpass filter that rejects frequencies
above 250kHz to reduce false counts due to high-speed transients.
This cutoff point allows encoder frequencies as high as one million
counts/second, because there are four counts per period, as shown
in Figure6.
Figure 6. Quadrature Encoder Counts/Period Relationship
Phase A
Phase B
00 10 11 01
1 period = 4 counts