54 Control Connections CG Drives & Automation, 01-5326-01r5
Fig. 65 Electro Magnetic (EM) screening of control signal
cables.
4.5.5 Current signals ((0)4-20 mA)
A current signal like (0)4-20 mA is less sensitive to
disturbances than a 0-10 V signal, because it is connected to
an input which has a lower impedance (250 Ω) than a
voltage signal (20 kΩ). It is therefore strongly advised to use
current control signals if the cables are longer than a few
metres.
4.5.6 Twisted cables
Analogue and digital signals are less sensitive to interference
if the cables carrying them are “twisted”. This is certainly to
be recommended if screening cannot be used. By twisting
the wires the exposed areas are minimised. This means that
in the current circuit for any possible High Frequency (HF)
interference fields, no voltage can be induced. For a PLC it
is therefore important that the return wire remains in
proximity to the signal wire. It is important that the pair of
wires is fully twisted over 360°.
4.6 Connecting options
The option cards are connected by the optional connectors
X4 or X5 on the control board see Fig. 56, page 47 and
mounted above the control board. The inputs and outputs
of the option cards are connected in the same way as other
control signals.
Pres-
sure
sensor
(exam-
ple)
External control
(e.g. in metal housing)
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