7-16
ET 200U Distributed I/O Station
EWA 4NEB 812 6087–02b
7.4
Analog Value Representation of Analog Input Modules
Each analog process signal has to be converted into a digital format to be
stored in the process image input table (PII). The analog signals are con-
verted into a binary digit that is written in one of the following ways:
In one byte (466–8MA11)
or
In two bytes (all other analog input modules)
Each bit position has a fixed value in powers of two (see Tables 7-4 and
7-14).
Analog values are represented in two’s complement.
Tables 7-5 through 7-11 show the analog value representations of different
analog inputs in two–byte format.
Table 7-4
Representation of an Analog Input Value as Bit Pattern
High Byte
Low Byte
Bit Number
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
Analog Value Represent.
S
2
11
2
10
2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
X
E
OV
S
Sign bit
0 = “+”
1 = “–”
X
Irrelevant bits
E
Error bit
0 = no wire break
1 = wire break
OV
Overflow bit
0 = Measured value 4095 units at the
most
1 = Measured value greater than or
equal to 4096 units
Analog Value Processing
