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Devices in Detail
4
Devices in Detail
The devices in PLCs are used directly in control program instructions. Their signal states can
be both read and changed by the PLC program. A device reference has two parts:
–
the device name and
–
the device address.
Example of a device reference (e.g. input 0):
Device name
4.1
Inputs and Outputs
The PLC's inputs and outputs connect it to the process that it is controlling. When an input is
polled by the PLC program the voltage on the input terminal of the controller is measured.
Since these inputs are digital they can only have two signal states, ON or OFF. When the volt-
age at the input terminal reaches 24V the input is on (state "1"). If the voltage is lower than 24V
the input evaluates as off (signal state "0").
In MELSEC PLCs the identifier "X" is used for inputs. The same input can be polled as often as
necessary in the same program.
NOTE
The PLC cannot change the state of inputs. For example, it is not possible to execute an OUT
instruction on an input device.
If an output instruction is executed on an output the result of the current operation (the signal
state) is applied to the output terminal of the PLC. If it is a relay output the relay closes (all relays
have make contacts). If it is a transistor output the transistor makes the connection and acti-
vates the connected circuit.
The identifier for output devices is "Y". Outputs can be used in logic operation instructions as
well as with output instructions. However, it is important to remember that you can never use an
output instruction on the same output more than once (see also section 3.4.2).
FX Beginners Manual
X 0
Device address
X000 X001
X002
Y000 Y001
Y002
The illustration on the left shows an example
of how you can connect switches to the
inputs and lamps and contactors to the out-
puts of a MELSEC PLC.
Inputs and Outputs
4 – 1
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