Using Switches And Sensors - Mitsubishi MELSEC System Q Manual

The Basic Instruction Set
4.7.3

Using switches and sensors

Before we continue with the description of the rest of the instructions we should first describe
how signals from switches, sensors and so on can be used in your programs.
PLC programs need to be able respond to signals from switches, buttons and sensors to per-
form the correct functions. It is important to understand that program instructions can only poll
the binary signal state of the specified input – irrespective of the type of input and how it is
controlled.
Usually, switches with make contacts are used. Sometimes, however, break contacts are used
for safety reasons – for example for switching off drives (see section 4.8).
The illustration below shows two program sequences in which the result is exactly the same,
even though different switch types are used: When the switch is operated the output is set
(switched on).
24 V
X0
24 V
X0
4 – 16
Normally
When a make normally open
open con-
contact is operated the input
tact (make
is set (ON, signal state "1")
contact)
Normally
closed When a normally closed con-
contact tact is operated the input is
(break reset (OFF, signal state "0")
contact)
X000
ON
X0
OFF
ON
Y10
OFF
X000
ON
X0
OFF
ON
Y10
OFF
An Introduction to Programming
As you can imagine, this means that when
you are writing your program you need to be
aware whether the element connected to the
input of your PLC is a make or a break device.
An input connected to a make device must be
treated differently to an input connected to a
break device. The following example illustra-
tes this.
Switch operated
Switch operated
MITSUBISHI ELECTRIC
Y010
t
Y010
t