Representation Of Analog Values; Overview - Siemens SIMATIC ET 200AL System Manual

Representation of analog values

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
Bit number
Significance of the bits
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 between+32767 and -32768 regardless of the resolution. The scaling
between two successive values depends on the resolution of the module.
60
Analog value
15 14 13 12
S 2 2 2
14 13 12
11 10 9 8 7
2 2 2 2 2
11 10 9 8
6 5 4 3
2 2 2 2
7 6 5 4 3
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
3
2 1 0
2 2 2
2 1 0