Interference-Free Connection Of Monitors - Siemens SIMATIC S7-400 Hardware And Installation Manual

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Interference-Free Connection of Monitors

Introduction
You can use operator interfaces with monitor ports from the COROS product
family. The layout of apparatus and the interference content of the environment
significantly affect the interference-free connection of monitors to a programmable
controller. The choice of monitor and video cables is governed by whether the
monitor and programmable controller are to be operated under low-interference
conditions or under industrial conditions.
Operation under Low-Interference Conditions
Where a monitor and the programmable controller operate in a low-interference
environment and there is only a short distance between monitor and programmable
controller, they are at almost the same ground potentials. Interference and
disturbance by ground loops are therefore not expected.
In these cases, you can use either TTL or analog signals to drive the monitor.
Digital cables or single-shielded coaxial cables can be used to transmit the video
signals. Note that the braided shield of the coaxial cable serves as the return
conductor and must not be connected to the shield bus. The monitor and
communications processor (CP) are interconnected without additional shielding
and grounding.
Operation under Industrial Conditions
Where the monitor and programmable controller are operated under harsh
industrial conditions or there is a great distance between monitor and
programmable controller, the apparatus may be at different ground potentials; this,
in turn, can result in interference and disturbances caused by ground loops.
In these cases, double-shielded coaxial cable (triaxial cable) must be used to
transmit the video signals. The inner braided shield of this cable serves as the
return conductor and must not be connected to the shield bus. The outer braided
shield serves to discharge interference currents and must be incorporated in
shielding and grounding measures.
To avoid ground loops, the electronics ground and housing ground of the monitor
must be separate. This requirement is considered to be met when one of the
following conditions is fulfilled:
Electronic and housing grounds of the monitor are isolated from each other.
Electronic and housing grounds of the monitor are interconnected via a
voltage-dependent resistor (VDR) fitted by the manufacturer of the monitor.
S7-400 and M7-400 Programmable Controllers, Installation Manual
C79000-G7076-C424-01
www.gkbpx.com
Assembling and Installing Systems
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