Technical data

www.westermo.com Theoretical and general applications 41
Ground networks
The very best overall method to minimise disturbances is for the system to
have an equipotential design. This means that buildings, electronics, fieldbuses
and field devices all have the same ground potential. This is very difficult to
achieve in practice, you can obtain a uniform potential with the help of special
ground conductors and ground wire networks. It is important that the ground
wire network and protective ground are interconnected and that they lie as
close to each other as possible.
Shielding
Shielded or double shielded cables can be used to increase the resistance to
external interference. Under normal circumstances the cable shield should
only be connected to ground at one end.
In some extreme circumstance where high frequency noise is a problem,
the cable can be connected to ground at both ends. However this method
introduces a potentially larger problem if there is a potential difference
between the points. If this is the case current will start to flow through the shield of
the cable and carry with it any noise on the ground plain.
As an alternative it is sometimes possible to connect one end of the shield to
ground and the other to ground via a small value, high voltage capacitor
Short Connections without a modem
Direct data communication using RS232/V.24 without a modem will only work over
very short distances. The cables must be routed separately from other cables, yet be
as close to the ground cable as possible. The device chassis should also be intercon-
nected using copper wire to reduce CMV (Common Mode Voltages) noise problems.
RS-232/V.24 provides slow communications over ranges up to a maximum of 15 m
(50 ft). A line driver or modem should be used for distances above 15 m (50 ft).
RS-422 provides better protection as both the transmitter and receiver are bal-
anced. Screened twisted pair cable can be used and devices must, if they are separate,
have their chassis interconnected and preferably fed from the same power source.
CMW=0
Data communications to
RS-422 for 10 Mbit.
Data communications to
RS-232/V.24.
B A C K