Rockwell Automation 2705 RediPanel Push Button Module - Manual (Page 58)

Chapter 4
Programming the RediPANEL Modules
4–19
The figure below illustrates a single operating scan or cycle of a PLC
processor.
End of program scan.
One cycle = Program scan + I/O scan
I/O scan: Status of inputs is noted and
outputs are energized.
Start of program scan. Instructions are
scanned sequentially, as entered.
On power up, the processor begins a scan of the user program.
•
The PLC scans the input/output (I/O) devices to determine their status.
•
During the I/O update scan, the PLC communicates with the scanner.
•
The scanner performs a remote I/O scan, asynchronous to the PLC’s scan.
The I/O chassis connected to the scanner are scanned in sequential order
according to the I/O rack number, regardless of I/O channel.
The remote I/O scan time varies according to the amount of I/O data change
taking place. Data from the input devices is transferred to the PLC’s input
image table and data from the PLC’s output image table is transferred to the
output devices. After completing the I/O scan, the processor begins the
program scan. At this time, all ladder logic program instructions are scanned
and executed in the order in which they were entered.
The I/O scan and the program scan are performed one after the other. The
operating cycle is the total time required for both the I/O scan and the
program scan.
The sections on Response Time and Handshake Mode cover the importance
of timing in the program portion of the cycle in contrast to the I/O portion.
Operating Cycles