Siemens Simatic S7-200 System Manual page 368

S7-200 Programmable Controller System Manual
Input Data Word Format for EM 231 and EM 235
Figure A-11 shows where the 12-bit data value is placed within the analog input word of the CPU.
Figure A-11
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 Diagram for EM 231 and EM 235
A+
RA
Rloop
A–
B+
RB
Rloop
B–
A
C+
RC
Rloop
C–
D+
RD
Rloop
D–
Input filter
Figure A-12
356
MSB
15 14
AIW XX 0
MSB
15
AIW XX
Input Data Word Format for EM 231 and EM 235
R
C
C
C
A=1
R
R
C
C
C
A=2
R
R
C
C
C
A=3
R
R
C
C
C
A=4
R
MUX 4 to 1
Input Block Diagram for the EM 231
Data value 12 Bits
Unipolar data
4 3
Data value 12 Bits
Bipolar data
EM 231
GAIN ADJUST
+
Instrumentation
AMP
–
LSB
3 2 0
0 0 0
LSB
0
0 0 0 0
BUFFER
A/D Converter
11 0