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6–1 Introduction
When writing and debugging a program, the timing of various operations
must be considered. Not only is the time required to execute the program
and perform other CPU operations important, but also the timing of each sig-
nal coming into and leaving the PC must be such that the desired control ac-
tion is achieved at the right time.
The major factors in determining program timing are the scan time and the
I/O response time. One cycle of CPU operation is called a scan; the time re-
quired for one cycle is called the scan time. The time required to produce a
control output signal following reception of an input signal is called the I/O
response time. This section explains the scan and shows how to calculate
the scan time and I/O response time.
6–2 Scan Time
To aid in PC operation, the average scan time can be displayed on the Pro-
gramming Console or any other Programming Device. Understanding the
operations that occur during the scan and the elements that affect scan time
is essential to effective programming and PC operations.
The overall flow of CPU operation is as shown in the following flowchart.
Scan Time Section 6–2
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