Siemens SIMATIC ET 200AL, System Manual

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Analog value processing
60
Function Manual, 06/2014, A5E03461439-AC
Representation of analog values
3
3.1 Overview
Conversion of analog values
The CPU only processes analog values in digitalized format.
Analog input modules convert the analog signal into a digital value for further processing by
the CPU.
Analog output modules convert the digital output value from the CPU into an analog signal.
Analog value representation at 16-bit resolution
The digitized analog value is the same for all I/O values at the same rated range. The analog
values are represented as fixed point numbers in twos complement. This leads to the
following conditions:
Table 3- 1 Representation of analog values
Resolution Analog value
Bit number 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Significance of the bits S 2
14
2
13
2
12
2
11
2
10
2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
Sign
The sign (S) of the analog value is always set in bit 15:
● "0" → +
● "1" → -
Resolution less than 16 bits
The analog value is left-aligned in the memory of analog modules with a resolution of less
than 16 bits. The unused least significant digits are padded with "0" values.
This reduces the number of measured values that can be represented. Modules occupy a
range of values of 32767 and -32768 regardless of the resolution. The scaling
between two successive values depends on the resolution of the module.