Table of Contents
Advertisement
With automation components, functional ground also ensures interference-free operation of
a controller. Via the functional ground, interference currents coupled in via the connecting
cables are discharged to ground.
In terms of the large-area and low impedance implementation, a functional ground set up for
this purpose generally also meets the requirements of protective earth. On the other hand,
protective earth does not always meet the requirements of functional ground. In general
while the connection for protective earth is always low resistance, it is not necessarily low
impedance.
The resistance of a connection for protective earth must always be as small as possible to
divert fault currents safely to ground. The length of a protective earth cable can therefore
be several meters (m) long, without seriously impairing this effect. For a functional ground
for diverting HF disturbances, this cable does however have impedance and is therefore not
suitable.
5.4
Digital input/output
The digital input and output are connected using a 2-pin plug-in terminal block in each case. Two
terminal blocks ship with the device.
NOTICE
Damage due to voltage being too high or too low
The voltage at the digital input/output must not exceed 30 VDC and not fall below -30 VDC,
otherwise the digital input/output will be destroyed.
Note
Interference pulse
To avoid evaluating an interference pulse, the pulse for the signal 1 (TRUE / HIGH) must be at
least 200 ms.
Rules for wiring
• To wire the digital input/output, use a copper cable of category AWG18-16 or a cable with a
cross-section of 0.75 to 1.5 mm
• Always wire the digital input/output in pairs.
• The maximum permitted cable length is 30 m.
SCALANCE M874, M876
Operating Instructions, 02/2023, C79000-G8976-C331-12
.
2
Connecting up
5.4 Digital input/output
61
Advertisement
Table of Contents
