Specifications

Analog Input Modules
31007715 6/2008 125
Data
Word Structure
The first and third STB ACI 1230 registers in the input block of the process image
are the data words. Each register represents the input current of a channel in the
IEC data format. The data has 12-bit resolution. The bit structure in each data
register is as follows:
There are 12 significant bits in each data word—bits 14 through 3. They allow you
to represent current data with integer values ranging from 0 to 32,760 in increments
of eight.
The value 32 000 represents 20 mA. If the input value exceeds 20 mA, the input
channel reports an over-current warning (OCW). If the input value reaches
20.48 mA, an over-current error (OCE) is reported. If the input value drops below
0.48 mA, the input channel reports an under-current warning (UCW), and if the value
is less than or equal to 0 the channel reports an under-current error (UCE).
Linear current representations can be interpreted using the formula:
where Cn is the numerical count and Iai is the analog input current.
The sign bit (bit 15) is always 0, indicating that negative values are not represented.
The three least significant bits in the data words are always 0.
2
14
2
13
2
12
2
11
2
10
2
9
2
8
2
7
2
6
2
5
2
4
2
3
always 0
always 0
8
0
STB ACI 1230 Data Register Format
12
3
45
67
910111213
1415
I
ai
C
n
1600⁄=
I
C
32 760
32 000
15 mA
20.48 mA
20 mA
24 000
OCE
ai
n
OCW
0 (UCE)
768
0.48 mA
UCW
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