Electrical Noise Control - Allen-Bradley 1395 Troubleshooting Manual

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Electrical Noise Control

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The system grounding configuration largely effects the integrity, and quality
of the controllers logic and low level analog signals. Other factors which
effect signal quality, and thus the integrity of the entire system, must also be
considered when troubleshooting 1395 systems.
Separation of low level control and high level switching / power conductors
must be maintained. Analog Reference signals, feedback signals (either
analog or digital), and serial communication wiring must be separated from
115V AC relay logic and any power wiring (brake, armature, field, external
relay wires, etc.). When wiring both inside and outside the 1395 system
cabinets it is desirable that low level signals be run in separate conduits
from power wiring. Shielded conductors must be used for all low level
analog signals, serial communication, and digital feedback signals
(encoder). This shielded cable should be of twisted pair type construction
with the shield tied to ground at the source end of the cable. The 1395
provides a shield bus connection TB4 at the bottom of the controller for
shield connections.
All inductive switching devices should be suppressed, at the device, to
decrease the DV/DT noise that is generated. A high rate of voltage change
in a conductor will induce a current in an adjoining coupler through
capacitive coupling. Two factors effect the amount of current induced:
1. The rate of change in voltage.
2. The relative distance between conductors.
By controlling both of these factors, induced system noise should be
minimized. This should help guard against nuisance faults and reduce stress
on CMOS devices used in the 1395 controller.
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