Programming Considerations for PLC-5 ProcessorsF–2
Publication
65566.5.1 - October 1996
How the Processor Scans the Program
The Processor:
1.Monitors Inputs
Monitors the status of input devices by means of input modules.
(The off/on or analog signal of each input appears as a
corresponding 0/1 or BCD bit pattern in the input image table.)
2.Controls Outputs
Controls the status of output devices by means of output modules.
(Sets outputs to an off/on or analog value according to the 0/1 or
BCD bit pattern stored in the output image table for the
corresponding output device.)
3.Executes Ladder Logic
Manipulates data, makes decisions, and/or controls outputs based
on the status of inputs according to your ladder logic instructions.
Time is the issue. A shorter program scan gives a faster system
response. Each of the following contributes to the time required for a
program scan.
•Instructions
•Processor Family
•Data format
•Addressing
Instructions
Each instruction contributes a small portion of time to the program
scan. The processor operates on some instructions very quickly, but
takes longer operating on others. Whether the instruction is true or
false also affects instruction execution time (Table F.A).
Affect Your Ladder Logic
Has on the Program Scan
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