Measurement Computing PCI-2511 User Manual page 19

PCI-2511 User's Guide
The concentric pattern for B has the same number of window pairs as A—except that the entire pattern is
rotated by 1/4 of a window-pair. Thus the B signal is always 90° out of phase from the A signal. The A and B
signals pulse 512 times (or 1024, 4096, etc.) per complete rotation of the encoder.
The concentric pattern for the Z signal has only one transparent window and therefore pulses only once per
complete rotation. Representative signals are shown in the following figure.
A
B
Z
Figure 12. Representation of quadrature encoder outputs: A, B, and Z
As the encoder rotates, the A (or B) signal indicates the distance the encoder has traveled. The frequency of A
(or B) indicates the velocity of rotation of the encoder. If the Z signal is used to zero a counter (that is clocked
by A) then that counter gives the number of pulses the encoder has rotated from its reference. The Z signal is a
reference marker for the encoder. It should be noted that when the encoder is rotating clockwise (as viewed
from the back), A leads B and when the encoder is rotating counterclockwise, A lags behind B. If the counter
direction control logic is such that the counter counts upward when A leads B and counts downward when A
lags B, then the counter gives direction control as well as distance from the reference.
Maximizing encoder accuracy
If there are 512 pulses on A, the encoder position is accurate to within 360°/512.
For a position accuracy of 360°/1024, count both rising edges on A and falling edges on A.
You get maximum accuracy when counting rising and falling edges on A and on B (since B also has 512
pulses.) This gives a position accuracy of 360°/2048.
These different modes are known as X1, X2, and X4.
Connecting the PCI-2511 to an encoder
You can use up to two encoders with each PCI-2511 in your acquisition system. Each A and B signal can be
made as a single-ended connection with respect to common ground.
Differential applications are not supported.
For single-ended applications:
Connect signals A, B, and Z to the counter inputs on the PCI-2511.

Connect each encoder ground to GND.

You can also connect external pull-up resistors to the PCI-2511 counter input terminal blocks by placing a pull-
up resistor between any input channel and the encoder power supply. Choose a pull-up resistor value based on
the encoder's output drive capability and the input impedance of the PCI-2511. Lower values of pull-up resistors
cause less distortion, but also cause the encoder's output driver to pull down with more current.
Connecting external pull-up resistors to the PCI-2511
For open-collector outputs, you can connect external pull-up resistors to the PCI-2511's counter input terminal
blocks. You can place a pull-up resistor between any input channel and the provided +5 V power supply.
Choose a pull-up resistor value based on the encoder's output drive capability and the input impedance of the
PCI-2511. Lower values of pull-up resistors cause less distortion but also cause the encoder's output driver to
pull down with more current.
Wiring to one encoder
2511. The "A" signal must be connected to an even-numbered channel and the associated "B" signal must be
connected to the next [higher] odd-numbered channel. For example, if "A" were connected to CTR0, "B" would
be connected to CTR1.
: Figure 13 shows the connections for one encoder to a 68-pin SCSI connector on a PCI-
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Functional Details