3 analog value representation – Siemens Simatic S7-400 User Manual
Page 223
Analog Modules
5-7
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
5.3
Analog Value Representation
Introduction
This section describes the analog values for all the measuring ranges and output
ranges which you can use with the analog modules.
Converting Analog Values
Analog input modules convert the analog process signal into digital form.
Analog output modules convert the digital output value into an analog signal.
Analog Value Representation with 16-Bit Resolution
The digitized analog value is the same for both input and output values having the
same nominal range. The analog values are represented as a fixed-point number
in two’s complement. The resulting assignment is as follows:
Bit
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Value of bits
2
15
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
Bit 15 Can Be Interpreted as a Sign
The sign of the analog value is always contained in bit number 15:
•
”0”
³
)
•
”1”
³
*
Resolution Less than 16 Bits
If the resolution of an analog module has fewer than 16 bits, the analog value is
stored left-justified on the module. The lower-order bit positions not used are
padded with zeros (“0”).