© National Instruments
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4-5
Encoder Connection Recommendations
The encoder inputs are connected to incremental 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 reduced motion system accuracy. The following additional
recommendations apply to encoder signal connections:
•
National Instruments strongly recommends you use encoders with differential line driver
outputs for all applications. You
must
use differential encoders if the encoder cable length
is longer than 3.05 m (10 ft). Shielded, 24 AWG (0.25 mm
2
cross section) wire is the
minimum recommended size for the encoder cable.
•
Wire encoder signals and their ground connections separately from all other connections.
Wiring these signals near the motor drive or other signals can cause positioning errors and
faulty operation.
•
Cables with twisted pairs and an overall shield are recommended for optimized noise
immunity. Figure 4-5 shows twisted pairs in a shielded cable. Unshielded cables can cause
noise to corrupt the encoder signals, resulting in lost or additional counts and reduced
motion system accuracy.
Figure 4-5.
Shielded Twisted Pairs
•
Tie the encoder cable shield to COM at the encoder side only.
Encoder A+
Encoder A–
Encoder B+
Encoder B–
Encoder Index+
Encoder Index–
+5 V OUT
COM
S
hield
Dr
a
in