National Instruments PXI-7340, User Manual And Specifications

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Chapter 4 Signal Connections
Encoder <1..4> Index (Phase Z)
The Index input is primarily used to establish a reference position. This function uses the number
of counts per revolution or the linear distance to initiate a search move that locates the index
position. When a valid Index signal transition occurs during a Find Reference routine, the
position of the Index signal is captured accurately. Use this captured position to establish a
reference zero position for absolute position control or any other motion system position
reference required.
The default MAX settings guarantee that the Find Index routine completes successfully if the
encoder generates a high index pulse when phases A and B are low and the encoder is connected
through an NI UMI or drive accessory. Figure 4-4 shows the default encoder phasing diagram at
the inputs to the controller.
Figure 4-4. Quadrature Encoder Phasing Diagram
You can set the index reference criteria in MAX to change the pattern of phases A and B for the
index search. You can also set the encoder polarity for phases A, B, and Z in MAX.
Wiring Concerns
The encoder inputs are connected to quadrature decoder/counter circuits. It is very important to
minimize noise at this interface. Excessive noise on these encoder input signals may result in
loss of counts or extra counts and erroneous closed-loop motion operation. Verify the encoder
connections before powering up the system.
Caution Wire encoder signals and their ground connections separately from all
other connections. Wiring these signals near the motor drive/amplifier or other
signals can cause positioning errors and faulty operation.
Encoders with differential line driver outputs are strongly recommended for all applications and
must be used if the encoder cable length is longer than 3.05 m (10 ft). Shielded, 24 AWG wire
is the minimum recommended size for the encoder cable. Cables with twisted pairs and an
overall shield are recommended for optimized noise immunity.
All National Instruments power drives and UMI accessories provide built-in circuitry that
converts differential encoder signals to single-ended encoder signals.
Caution Unshielded cable can cause noise to corrupt the encoder signals, resulting
in lost counts and reduced motion system accuracy.
Ph as e A
Ph as e B
Index