Rockwell Automation 8520 9/Series CNC Integration Maintenance Manual Documentation Set - Manual (Page 420)

Section 7C
Power Distribution and Wiring Guidelines
7C-2
Important: When grounding more than one electronic device, do not
serially connect their grounding wires. If connected in series, even low
level noises can cause interference, as they accumulate additively.
Grounding must be made to a single point.
Mechanical Shielding
Radiated noise generates high frequency noise on high-impedance control
inputs. For effective noise prevention, install a low-impedance grounded
conductor in the radiated noise transmission route. Normally, installing the
control inside a metallic cabinet and providing protective ground to this
cabinet will produce satisfactory results.
Shielded Cables and Twisted-pair Cables
Shielded cables are very effective for protecting weak signals such as
control signals from electrical noise.
These cables are effective for protecting the signals from full frequency
band electrostatic inductive noises and radio frequency band
electromagnetic noises. However, they are not as effective for low
frequency magnetic inductive noises and electromagnetic inductive noise
of frequencies lower than radio frequency band.
Generally, signal cables carrying signals of similar voltage or current levels
are run along the same route. This sometimes causes electrostatic coupling
between the signal cables. To prevent such a problem, use shielded cables
to carry weak signals, drive signals, and feedback signals, for example.
Exposure of signal cables must be kept to a minimum to provide common
mode noise rejection and protection. Ground the shielded cable only at
one end (usually, the controller or signal source end). The other end of the
shield cable should be insulated with tape or shrink tubing.
Important: If both ends of the shielded cable are connected to ground, a
ground loop will be formed. This may conduct the noise current through
the cabinet frame and the chassis ground, generating noise problems
instead of eliminating them.
To carry NC control signals, use twisted--pair cables, as shown in
Figure 7C.1, to prevent transmission of noise to the differential input
signals.