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Processing Analog Values
7.1.1
Criteria for selecting analog modules
A wide range of analog modules are available for the MELSEC FX family and you need to
choose the correct module for each automation task. The main criteria for selection are as
follows:
b Compatibility with the PLC base unit
The analog module must be compatible the with PLC base unit you are using. For example,
you cannot connect the analog modules of the FX
series.
b Resolution
The resolution describes the smallest physical value that can be acquired or output by the
analog module.
In the case of analog input modules the resolution is defined as the change in voltage, cur-
rent or temperature at the input that will increase or decrease the digital output value by 1.
In the case of analog output modules the resolution is the change in the voltage or current
value at the module output caused by increasing or decreasing the digital input value by 1.
The resolution is restricted by the internal design of the analog modules and depends on
the number of bits required to store the digital value. For example, if a 10V voltage is
acquired with a 12-bit A/D converter the voltage range is divided into 4,096 steps (2
4096, see section 3.3). This results in a resolution of 10V/4096 = 2.5mV.
b Number of analog inputs or outputs
The inputs or outputs of analog modules are also referred to as channels. You can choose
analog input modules with 2, 4 or 8 channels, depending on the number of channels you
need. Please note that there is a limit to the number of special function modules you can
connect to a PLC base unit (see section 7.1.2). If you know that you will need to install other
special function modules it is thus better to use one module with four channels rather than
two modules with two channels each, because this allows you to connect more additional
modules.
FX Beginners Manual
Base unit of
FX family
Digital value
e. g. 2000
Analog output
module
Voltage or
current
Digital/Analog
conversion
z. B. 5 V
oder 12 mA
The level of the current or vol-
tage signal from the PLC con-
trols the speed of the connected
motor.
series to a base unit of the FX
3U
Analog Modules
Inverter
1N
12
=
7 – 3
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