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19
Basic points to remember:
- Writing NOP instructions in the middle of a program minimizes step number changes
when changing or editing a program.
- It is possible to change the operation of a circuit by replacing programmed
instructions with NOP instructions.
- Changing a LD, LDI, ANB or an ORB instruction with a NOP instruction will change
the circuit considerably; quite possibly resulting in an error being generated.
- After the program ‘all clear operation’ is executed, all of the instructions currently in
the program are over written with NOP’s.
20. End
Basic points to remember:
- Placing an END instruction in a program forces that program to end the current scan
and carry out the updating processes for both inputs and outputs.
- Inserting END instructions in the middle of the program helps program debugging as
the section after the END instruction is disabled and isolated from the area that is
being checked. Remember to delete the END instructions from the blocks which have
already been checked.
- When the END instruction is processed the PCs watchdog timer is automatically
refreshed.
A program scan:
• A program scan is a single processing of the loaded program from start to finish,
This includes updating all inputs, outputs and watchdog timers. The time period for
one such process to occur is called the scan time. This will be dependent upon
program length and complexity. Immediately the current scan is completed the next
scan begins. The whole process is a continuous cycle. Updating of inputs takes place
at the beginning of each scan while all outputs are updated at the end of the scan.
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