Measurement Computing USB-1602HS-2AO - Manual (Page 27)

USB-1602HS-2AO User's Guide Functional Details
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Figure 19. 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 will lead 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, then the encoder position is accurate to within 360°/512.
You can get even greater accuracy by counting not only rising edges on A but also falling edges on A, giving
position accuracy to 360°/1024.
You get maximum accuracy 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 USB-1602HS-2AO to an encoder
You can connect up to three quadrature encoders to the USB-1602HS-2AO. Quadrature encoders with a 16-bit
(counter low) or a 32-bit (counter low + counter high) counter, 6 MHz maximum pulse frequency, and X1, X2,
and X4 count modes are supported.
The SCSI connector provides six counter pins that are dedicated to performing encoder functions:
 CTR4 A, CTR4 B, CTR4 Z
 CTR5 A, CTR5 B, CTR5 Z
 CTR6 A, CTR6 B, CTR6 Z
The encoder input pins are dedicated for encoder counting, and cannot be mapped for other functions. Input A is
a direct counter input. Input B functions as the B phase for encoder counting, and Input Z functions as the
Z phase of the encoder.
Each A and B signal is made as a single-ended connection with respect to common ground. To connect an
encoder to the USB-1602HS-2AO, do the following:
 Connect signals A, B, and Z to the CTR4 A, B, and Z through CTR6 A, B, and Z pins on the SCSI
connector.
 Connect each encoder ground to GND.
Figure 20 shows the connections for one encoder to the 68-pin SCSI connector on a USB-1602HS-2AO.
A
B
Z