National Instruments 7340 PCI Benutzerhandbuch

© National Instruments Corporation

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.

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.

Unshielded cable can cause noise to corrupt the encoder signals, resulting in lost 

counts and reduced motion system accuracy.

Figure 5-4 shows a simplified schematic diagram of the circuit used for 
the Phase A, Phase B, and Index encoder inputs. Both phases A and B are 
required for proper encoder counter operation, and the signals must support 
the 90° phase difference within system tolerance. The encoder and Index 
signals are conditioned by a software-programmable digital filter inside 
the FPGA. The Index signal is optional but highly recommended and 
required for initialization functionality with the Find Index function.