Siemens SIMATIC S7-400 Reference Manual page 168

Analog modules
5.4 Representation of analog values
Representation of analog values at a resolution of 16 bits
The digitized analog value applies to input and output values of the same nominal range.
Analog values are output as real numbers in two's complement. The resulting assignment:
Bit
Bit values
Bit 15 can be interpreted as a sign
The sign of the analog value is always set at bit 15:
● "0" → +
● "1" → -
Resolution less than 16 bits
On analog modules with a resolution of less than 16 bits, the analog value is stored left-justified.
The unused least significant bit positions are padded with "0".
Example
The example below demonstrates the "0" padding of unused bit positions for low resolution
values.
Table 5-4
Bit
16-bit analog value
13-bit analog value
168
15 14 13 12
2 2 2 2
15 14 13
Example: Bit pattern of a 16-bit and 13-bit analog value
Resolution
15 14 13 12 11 10 9
0 1
0 1
11 10 9 8
2 2 2 2
12 11 10 9 8
Analog value
0 0 0 1 1
0 0 0 1 1
Reference Manual, Ausgabe 11/2016, A5E00850736-08
7 6 5 4 3
2 2 2 2 2
7 6 5 4 3
8 7 6 5 4 3
0 0 1 1 1 0
0 0 1 1 1 0
S7-400 Automation System Module Data
2 1 0
2 2 2
2 1 0
2 1 0
0 1 1
0 0 0