Siemens S7-400 Interference-Free Connection of Monitors, Operation under Industrial Conditions

Assembling and Installing Systems

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.

A-34	Automation System S7-400	Hardware and Installation
		A5E00850741-01