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FX Series Programmable Controllers Execution Times And Instructional Hierarchy 7
7-14
7.4 Batch Processing
This is the s
y
stem used b
y
all members of the FX famil
y
of PLC’s. The basic concept is that
there are three sta
g
es to an
y
pro
g
ram scan. In other words, ever
y
time the pro
g
ram is
processed form start to end the followin
g
sequence of events occurs:
Input processing:
All of the current input statuses are read in to a
temporar
y
memor
y
area; sometimes called an
ima
g
e memor
y
. The PLC is now read
y
for the next
pro
g
ram processin
g
.......
Program processing:
All of the updated inputs are checked as the
pro
g
ram is processed. If the new input statuses
chan
g
e the status of driven outputs, then these are
noted in the ima
g
e memor
y
for the......
Output processing:
The new, current statuses of the outputs which
have
j
ust be processed are ph
y
sicall
y
updated, i.e
rela
y
s are turned ON or OFF as required. The
pro
g
ram scan starts a
g
ain............
The s
y
stem is known as 'Batch processin
g
'
because all of the inputs, pro
g
ram operation and finall
y
the outputs are processed as batches.
7.5 Summary of Device Memory Allocations
The memor
y
allocations of the pro
g
rammable are ver
y
complex, but from a users point of view
there are three main areas:
a) The Pro
g
ram Memor
y
:
This memor
y
area holds all of the data re
g
ardin
g
: parameters, sequence pro
g
ram, constant
values K and H, pointer information for P and I devices, nest level information, file re
g
ister
contents/allocations and also the pro
g
ram comment area.
- This memor
y
area is latched either b
y
batter
y
backup or b
y
use of EEPROM pro
g
ram
mana
g
ement (dependent on the PLC bein
g
used). An
y
data stored in this area is kept
even when the PLC is powered down. The duration and reliabilit
y
of the data stora
g
e is
dependent upon the condition of the batter
y
or EEPROM bein
g
used to perform the
backup process.
Input Processing
Program Processing
Output Processing