DL205 Analog Manual, 7th Edition Rev. D
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Chapter 6: F2-04RTD 4-Channel RTD Input
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Negative Temperature Readings with Magnitude Plus Sign for the DL240, DL250-1
and DL260 CPUs (Pointer Method)
With bipolar ranges, some additional logic will be needed to determine whether the value being
returned represents a positive voltage or a negative voltage. For example, the direction for a
motor might need to be known. There is a solution for this:
•
If bipolar ranges are used and a value greater than or equal to 8000
hex
is obtained, the value is
negative.
•
If a value less than or equal to 7FFF
hex
is obtained, then the value is positive.
The sign bit is the most significant bit, which combines 8000
hex
to the data value. If the value is
greater than or equal to 8000
hex
, only the most significant bit and the active channel bits need
to be masked to determine the actual data value.
NOTE:
DL240 CPUs with firmware release version 2.5 or later and DL250 CPUs with firmware release version 1.06 or
later support this method. Use the DL230 multiplexing example if your firmware is an earlier version.
The two programs on the next page show how this can be accomplished. The first example uses
magnitude plus sign (binary) and the second example uses magnitude plus sign (BCD). The
examples only show two channels.
It is good to know when a value is negative, so these rungs should be placed before any other
operations that use the data, such as math instructions, scaling operations, etc. Also, if stage
programming instructions are being used, these rungs should be in a stage that is always active.
NOTE:
This logic is only needed for each channel that is using bipolar input signals.
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Summary of Contents for DL205
Page 1: ...DL205 Analog I O Manual Manual Number D2 ANLG M ...
Page 2: ...Notes ...
Page 6: ...Notes ...
