Analog Modules
4-160
Programmable Logic Controllers S7-300 Module Data
A5E00105505-03
4.25.2 Measuring Methods and Measuring Ranges of the SM 331;
AI 2 x 12 bits
Measuring Methods
You can set the following measuring methods for the input channels:
• Voltage measurement
• Current measurement
• Resistance test
• Temperature measurement
You perform the setting by means of the measuring range module on the module
and with the “measuring method” parameter in STEP 7.
Unused Channels
You must short-circuit unused channels and connect them to M
ANA
. In this way,
you obtain an optimum interference immunity for the analog input module. Set the
“measuring method” parameter for unused channels to “disabled”. In this way you
shorten the scan time of the module.
If you do not use the COMP input, you must short-circuit it also.
Special characteristics of unused channels for some measuring ranges
Since configured inputs can remain unused because of the channel group
generation, you must take note of the following special characteristics of these
inputs to enable the diagnostic functions on the used channels.
• Measuring range 1 to 5 V: Connect the unused input in parallel with a used
input of the same channel group.
• Current measurement, 2-wire transmitter:
There are two ways to use the channels:
a) Leave the unused input open and do not enable diagnostics for this
channel group. Otherwise, if diagnostics are enabled, the analog module
triggers a single diagnostic interrupt and the SF LED of the analog module
comes on.
b) Connect the unused input with a resistance of 1.5 to 3.3 k You may then
enable diagnostics for this channel group.
• Current measurement 4 to 20 mA, 4-wire transmitter: Connect the unused
input in series with an input of the same channel group.
Measuring ranges
You perform the setting of the measuring ranges by means of the measuring range
module on the module and with the “measuring method” parameter in STEP 7.
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