AutomationDirect DL205 - Filtering Input Noise (DL250-1 and DL260 Cpus Only)

DL205 Analog Manual, 7th Edition Rev. D
7-23
Chapter 7: F2-04THM, 4-Channel Thermocouple Input
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Filtering Input Noise (DL250-1 and DL260 CPUs Only)
Add the following logic to filter and smooth analog input noise in DL250-1 and DL260 CPUs.
This is especially useful when using PID loops. Noise can be generated by the field device and/
or induced by field wiring.
The analog value in BCD is first converted to a binary number. Memory location V1400 is the
designated workspace in this example. The MULR instruction is the filter factor, which can be
from 0.1–0.9. The example uses 0.2. Using a smaller filter factor increases filtering. A higher
precision value can be used, but it is not generally needed. The filtered value is then converted
back to binary and then to BCD. The filtered value is stored in location V1402 for use in the
application program or a PID loop.
NOTE: Be careful not to do a multiple number conversion on a value. For example, if the pointer method is used to get
the analog value, it is in BCD and must be converted to binary. However, if the conventional method of reading analog is
used and the first 15 bits are masked, the value is already in binary and no conversion is needed. Also, if the conventional
method is used, change the LLD V2000 instruction to LD V2000.
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LDD
V2000
SUBR
V1400
BTOR
SP1
BIN
Loadsthe analog signal, whichisaBCDvalue
and has beenloadedfromV-memorylocation
V2000,intothe accumulator.Contact SP1 is
always on.
Converts theBCD value in theaccumulator to
binary. Remember,thisinstruction is not
neededifthe analog value is originally
broughtinasabinary number.
Converts thebinaryvalue in theaccumulator
toarealnumber.
Subtractsthe real numberstoredinlocation
V1400 from thereal numberinthe accumulator,
and stores theresultinthe accumulator.V1400
is thedesignatedworkspaceinthisexample.
Multipliesthe real number in the
accumulatorby0.2 (the filter factor),
and stores theresultinthe
accumulator.This is thefiltered value.
OUTD
V1400
ADDR
V1400
MULR
R0.2
OUTD
V1402
BCD
RTOB
Adds thereal numberstoredin
locationV1400 to therealnumber
filtered valueinthe accumulator, and
stores theresult in theaccumulator.
Copies thevalue in theaccumulator to
location V1400.
Converts thereal number in the
accumulatortoabinaryvalue, and
stores theresult in theaccumulator.
Converts thebinaryvalue in theaccumulator
to aBCD number.
NOTE: The BCD instruction is not needed for
PID loop PV (loop PV is a binary number).
Loads theBCD number filtered valuefrom
theaccumulator intolocationV1402touse in
your applicationorPID loop.