Siemens Simatic S7-400, Руководство по установке

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Assembling and Installing Systems
A-34
Automation System S7-400 Hardware and Installation
A5E00850741-01
A.12 Interference-Free Connection of Monitors
Introduction
You can use control and monitoring interfaces with monitor ports from the WinCC
product family. The layout of apparatus and degree of interference are decisive
factors for the interference-free connection of monitors to a PLC. A decisive factor
in the selection of monitor and video cables is the location of the monitor and PLC,
that is, depending on whether these are operated under low-interference or
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 signal. 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.
In order to avoid ground loops, electronics ground and chassis ground of the
monitor must be isolated. This is achieved by satisfying the following requirement:
•
Electronic and chassis grounds of the monitor are isolated from each other.
•
Electronic and chassis grounds of the monitor are interconnected via a
voltage-dependent resistor (VDR) fitted by the manufacturer of the monitor.