Scan Cycle Flashcards
(12 cards)
Initialize various memory based on retentive configuration
Non retentive memory is reset to zero while retentive areas are preserved
Update input
The CPU reads the status of the inputs, they stores it in an input images table
Service peripheral
Primarily a communications service
Peripheral devices such as remote programmers are read to establish if any updating is required because of bit forcing
Update special relays
Special bits and relays such as used as error flags are updated
Execute program
The ladder logic is solved and an output image table is produced with the results
Ladder evaluated from top to bottom, left to right
Update outputs
The outputs are updated as per the output image table
Do diagnostics
After diagnostics are performed the loop(except power section) is performed again
Scan time
The time taken for the CPU to complete the cycle
The only part of the scan cycle we have control over
The section where the ladder logic is solved
How to minimize the scan cycle
- Writing the smallest and most efficient program possible
- Using instructions which are evaluated quicker
Or
Use immediate instructions with high speed inputs
Or
Split the program up into two using separate CPU
What if the scan time is so long?
What can be done?
Inputs will be missed
Immediate instructions may be used with high speed inputs
Split the program up into two separate CPUs for each
What is scan cycle
Strict sequential operation
