DL205 Analog Manual, 7th Edition Rev. D
7-15
Chapter 7: F2-04THM, 4-Channel Thermocouple 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 following two programs on this page and 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.
Magnitude Plus Sign (Binary)
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b
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SP1
OUT
C1
LD
V2000
OUT
V2010
Load channel 1 data from V-memory into the
accumulator. Contact SP1 is always on.
Put the actual signal value in V2010. Now you can use
the data normally.
ANDD
K7FFF
This instruction masks the sign bit of the binary data, if
it is set. Without this step, negative values will not be
correct so do not forget to include it.
V2000
K8000
Channel 1 data is negative when C1 is on (a value of
--1.0 reads as 8010, --2.0 is 8020, etc.).
SP1
OUT
C2
LD
V2002
OUT
V2012
Load channel 2 from V-memory into the accumulator.
Contact SP1 is always on.
Put the actual signal value in V2012. Now you can use
the data normally.
ANDD
K7FFF
This instruction masks the sign bit of the binary data, if
it is set. Without this step, negative values will not be
correct so do not forget to include it.
V2002
K8000
Channel 2 data is negative when C2 is on (a value of
--1.0 reads as 8010, --2.0 is 8020, etc.).
²
²
Check Channel 1
Check Channel 2
Summary of Contents for DL205
Page 1: ...DL205 Analog I O Manual Manual Number D2 ANLG M ...
Page 2: ...Notes ...
Page 6: ...Notes ...
