Scan Cycle

Question 1

Initialize various memory based on retentive configuration

Answer 1

Non retentive memory is reset to zero while retentive areas are preserved

Question 2

Update input

Answer 2

The CPU reads the status of the inputs, they stores it in an input images table

Question 3

Service peripheral

Answer 3

Primarily a communications service
Peripheral devices such as remote programmers are read to establish if any updating is required because of bit forcing

Question 4

Update special relays

Answer 4

Special bits and relays such as used as error flags are updated

Question 5

Execute program

Answer 5

The ladder logic is solved and an output image table is produced with the results
Ladder evaluated from top to bottom, left to right

Question 6

Update outputs

Answer 6

The outputs are updated as per the output image table

Question 7

Do diagnostics

Answer 7

After diagnostics are performed the loop(except power section) is performed again

Question 8

Scan time

Answer 8

The time taken for the CPU to complete the cycle

Question 9

The only part of the scan cycle we have control over

Answer 9

The section where the ladder logic is solved

Question 10

How to minimize the scan cycle

Answer 10

1. Writing the smallest and most efficient program possible
2. Using instructions which are evaluated quicker