Siemens SIMATIC S7-200 System Manual page 436

S7-200 Programmable Controller System Manual
Installation Guidelines
Use the following guidelines to ensure accuracy and repeatability:
Ensure that the 24-VDC Sensor Supply is free of noise and is stable.
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Use the shortest possible sensor wires.
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Use shielded twisted pair wiring for sensor wires.
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Use a braided shield for best noise immunity.
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Terminate the shield at the Sensor location only.
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Short the inputs for any unused channels, as shown in Figure A-21.
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Avoid bending the wires into sharp angles.
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Use wireways for wire routing.
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Avoid placing signal wires parallel to high-energy wires. If the two wires must meet, cross
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them at right angles.
Ensure that the input signals are within the common mode voltage specification by isolating
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the input signals or referencing them to the external 24V common of the analog module.
Tip
The EM 231 and EM 235 expansion modules are not recommended for use with
thermocouples.
Understanding the Analog Input Module: Accuracy and Repeatability
The EM 231 and EM 235 analog input modules are low-cost, high-speed 12 bit analog input
modules. The modules can convert an analog signal input to its corresponding digital value in
149 µsec. The analog signal input is converted each time your program accesses the analog
point. These conversion times must be added to the basic execution time of the instruction used
to access the analog input.
The EM 231 and EM 235 provide an unprocessed
digital value (no linearization or filtering) that
corresponds to the analog voltage or current presented
at the module's input terminals. Since the modules are
high-speed modules, they can follow rapid changes in
the analog input signal (including internal and external
noise).
You can minimize reading-to-reading variations caused
by noise for a constant or slowly changing analog input
signal by averaging a number of readings. Note that
increasing the number of readings used in computing
the average value results in a correspondingly slower the average value results in a correspondingly slower
response time to changes in the input signal.
Figure A-22 shows the 99% repeatability limits, the mean or average value of the individual
readings, and the mean accuracy in a graphical form.
The specifications for repeatability describe the reading-to-reading variations of the module for an
input signal that is not changing. The repeatability specification defines the limits within which 99%
of the readings will fall. The repeatability is described in this figure by the bell curve.
The mean accuracy specification describes the average value of the error (the difference between
the average value of individual readings and the exact value of the actual analog input signal).
Table A-23 gives the repeatability specifications and the mean accuracy as they relate to each of
the configurable ranges.
422
Average Value
Repeatability limits
(99% of all readings fall within these limits)
Figure A-22 Accuracy Definitions
Signal Input
Mean
(average)
Accuracy