Siemens SIMATIC S7-200 System Manual page 433

Input Data Word Format for EM 231 and EM 235
Figure A-16 shows where the 12-bit data value is placed within the analog input word of the CPU.
Figure A-16 Input Data Word Format for EM 231 and EM 235
Tip
The 12 bits of the analog-to-digital converter (ADC) readings are left-justified in the data word
format. The MSB is the sign bit: zero indicates a positive data word value.
In the unipolar format, the three trailing zeros cause the data word to change by a count of eight
for each one-count change in the ADC value.
In the bipolar format, the four trailing zeros cause the data word to change by a count of sixteen
for each one count change in the ADC value.
Input Block Diagrams for EM 231 and 235
A+
R
RA C
C
Rloop
C
A--
R
B+
R
RB
C
C
Rloop
C
B--
R
C+
R
RC
C
C
Rloop
C
C--
R
D+
R
C
RD
C
Rloop
C
D--
R
MUX 4 to 1
Input filter
Figure A-17 Input Block Diagram for the EM 231 Analog Input, 4 Inputs
MSB
15 14
AIW XX 0
MSB
15
AIW XX
EM 231Analog Input, 4 Inputs
A=1
+
Instrumentation
AMP
--
A=2
A=3
A=4
Technical Specifications
Data value 12 Bits
Unipolar data
Data value 12 Bits
Bipolar data
GAIN ADJUST
Appendix A
LSB
3 2 0
0 0 0
LSB
4 3 0
0 0 0 0
BUFFER
A/D Converter
11 0
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