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Safety First!
The example on the right shows such an inter-
lock with contactor contacts. Here it is physi-
cally impossible for contactors K1 and K2 to be
switched on at the same time.
Automatic shutdown
When a PLC is used to control motion sequences in which hazards can arise when compo-
nents move past certain points additional limit switches must be installed to interrupt the move-
ment automatically. These switches must function directly and independently of the PLC. See
Chapter 3.6.2 for an example of such an automatic shutdown facility.
Output signal feedback
Generally, the outputs of PLCs are not monitored. When an output is activated the program
assumes that the correct response has taken place outside the PLC. In most cases no addi-
tional facilities are required. However, in critical applications you should also monitor the out-
put signals with the PLC – for example when errors in the output circuit (wire breaks, seized
contacts) could have serious consequences for safety or system functioning.
In the example on the right a make contact in
contactor K1 switches input X002 on when out-
put Y000 is switched on. This allows the pro-
gram to monitor whether the output and the
connected contactor are functioning properly.
Note that this simple solution does not check
whether the switched equipment is functioning
properly (for example if a motor is really turn-
ing). Additional functions would be necessary
to check this, for example a speed sensor or a
voltage load monitor.
3 – 22
An Introduction to Programming
X000 X001
X002
COM Y000
Y001
K2
K1
K1
K2
X000 X001
X002
+24 V
COM Y000
Y001
K1
MITSUBISHI ELECTRIC
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