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FX Series Programmable Controllers

7-14

This is the system used by all members of the FX family of PLC’s. The basic concept is that
there are three stages to any program scan. In other words, every time the program is
processed form start to end the following sequence of events occurs:

All of the current input statuses are read in to a
temporary memory area; sometimes called an
image memory. The PLC is now ready for the next
program processing.......

All of the updated inputs are checked as the
program is processed. If the new input statuses
change the status of driven outputs, then these are
noted in the image memory for the......

The new, current statuses of the outputs which
have just be processed are physically updated, i.e
relays are turned ON or OFF as required. The
program scan starts again............
The s ystem is known as 'Batch  pr ocessin g'

because all of the inputs, program operation and finally the outputs are processed as batches.

The memory allocations of the programmable are very complex, but from a users point of view
there are three main areas:

a) The Program Memory:

This memory area holds all of the data regarding: parameters, sequence program, constant
values K and H, pointer information for P and I devices, nest level information, file register
contents/allocations and also the program comment area.

-

This memory area is latched either by battery backup or by use of EEPROM program
management (dependent on the PLC being used). Any data stored in this area is kept
even when the PLC is powered down. The duration and reliability of the data storage is
dependent upon the condition of the battery or EEPROM being used to perform the
backup process.

FX

0(S)

FX

0N

FX FX

(2C)

FX

2N(C)

Input Processing

Program Processing

Output Processing