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Analog Inputs and Outputs
Many PLCs also work with analog I/O devices. Analog devices
use signals that are continuously variable within a specified
range, such as 0 to 10 VDC or 4 to 20 mA.
Analog signals are used to represent variable values, such as
speed, rate of flow, temperature, weight, level, etc. In order to
process an input of this type, a PLC must convert the analog
signal to a digital value. S7-200 PLCs convert each analog
voltage or current value into a 12-bit digital value.
Digital values from analog inputs are stored in addressable
memory for use by the user program. Similarly, the user
program can place digital values in addressable memory
locations for conversion to analog values for the designated
analog outputs.
The only S7-200 CPU model with analog I/O points on board
is CPU 224XP, which has 2 analog inputs and 1 analog output.
However, analog I/O points can be added using expansion
modules for any CPU other than CPU 221. CPU 222 allows
for 2 expansion modules and the remaining CPUs allow for 7
expansion modules.
Expansion modules are available with 4 or 8 analog inputs,
2 or 4 analog outputs, or 4 analog inputs and 1 analog
output. In addition, expansion modules are available for use
with thermocouples or RTD type sensors which sense the
temperature at a specific point in a machine or process.
Analog Expansion Module
SF/DIAG
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