CTI 2500 Installation and Operation Guide V2.8
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CHAPTER 6
SCAN TIMELINE
6.1 Scan Overview
The controller executes most tasks in a cyclical sequential order. This order is referred to as the
controller scan. There are two major divisions to the CTI 2500 controller scan; the discrete scan
and the analog scan. The discrete scan contains elements commonly found in traditional
programmable logic controllers. The analog scan provides an execution framework better suited
to process control tasks such as PID loops and analog alarms.
Figure 1. Controller Scan
6.1.1 Discrete Scan
The discrete scan consists of three tasks. These tasks always run to completion.
•
Normal I/O
During the Normal I/O task, I/O register output values are transferred to the physical I/O and
new input values are obtained from the physical I/O and written to the I/O register. All
configured I/O points are fully updated each scan.
•
RLL Execution
During the Main RLL Execution task, the Relay Ladder Logic is evaluated and executed.
The I/O register is updated with values derived by the RLL. Physical I/O may be read or
written during the RLL execution by using immediate I/O instructions.
•
Special Function I/O
Discrete
Scan
Analog
Scan
Normal I/O
Update
Main
RLL
SF I/O
