Laurel Electronics LAUREATE SERIES COUNTER_TIMER - Manual (Page 36)

13.3 ZERO INDEX SETUP
The relationship between the zero index correction signal and the Channels A & B signals varies
with different encoder model numbers and different manufacturers. To accommodate this
variation, the Quadrature board has control jumpers and selectable outputs that provide
ANDing of the zero index signal with all possible combinations of the Channel A & B signals.
Consider a typical encoder model that produces the waveforms shown below. Assume X4
counting is selected. The count increases and decreases on each A & B transition and remains
steady between transitions. The counts shown below the waveforms represent the effect of
the zero index correction ZI if no ANDing is used. Note the difference in count in the regions
between transitions when counting up and then counting down. The zero index correction is
made on the leading edge transition of the zero index signal. When counting down, the leading
edge is the trailing edge of the signal shown below because time is increasing from right to left.
A Channel
B Channel
Zero Index
Up
-3 -2 -1 0 1 2 3 4 5
ZI
Down -6 -5 -4 -3 -2 -1 0 3 4
ZI
| | | | | | | |
Up Region -3 -2 -1 0 1 2 3 4 5
Down Region -5 -4 -3 -2 -1 0 3 4 5
It follows that a wide zero index signal causes a discrepancy in the count in the regions between
transitions when counting up and counting down. To correct this situation, AND the zero index
signal with the A & B channel signals. Assume for this example the zero index is ANDed with
the inverse of A (/A) and the inverse of B (/B) to produce ZIY as shown below.
A Channel
B Channel
ZIY
Zero index AND ZIY = ZIR
Up
-3 -2 -1 0 1 2 3 4
ZIR
Down -4 -3 -2 -1 0 1 2 3
ZIR
| | | | | | | |
Up Region -3 -2 -1 0 1 2 3 4
Down Region -3 -2 -1 0 1 2 3 4
By ANDing the zero index signal with the A & B channels, there is no regional discrepancy
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