CTI 2500 Installation and Operation Guide V2.8
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Special Function programs called by analog loops or alarms will be executed during the calling
loop or alarm task time slice. Similarly, Special Function subroutines called by Special Function
programs or other Special Function subroutines shall be executed during the calling program
time slice.
Non-Cyclic Analog Tasks
Non-cyclic analog tasks are “called” from ladder, or are otherwise event based. They include:
•
Priority and Normal SF programs
These programs are queued for execution when the SFPGM RLL box receives power
flow. The processes are executed in the order in which they are queued. There is a
separate task for priority SF programs and normal SF programs.
•
RLL Requested SF Subroutines
These subroutines are queued for execution when SFSUB RLL box receives power flow.
There is a separate task for SFSUB and SFSUB0 instructions requests.
•
Normal Communications
This task processes deferred requests that require several scans to complete, such as
program edit operations. These requests may originate from the local serial port, USB
port, local Ethernet port, or from Special Function modules.
•
Priority Communications
This task
processes all requests from the serial and USB ports that can be processed in
a single scan.
•
Network Communications
This task processes all requests from the local Ethernet port, except deferred requests.
•
Run-time diagnostics
Run-time diagnostics are executed continuously over several scans. The diagnostic task
is allocated a fixed time slice.
NOTE
Ladder Initiated PID Loops and Special Function Programs and Subroutines that are marked for In-Line
execution are executed immediately during the RLL portion of the Scan.
6.1.3 Cyclic RLL
The Cyclic RLL is executed on a user-specified interval. When queued for execution, it will
interrupt any other task in the discrete scan or the analog scan. See the CTI 2500 Programming
Reference Manual for additional information.
6.2 PLC Scan Types
The CTI 2500 controller provides three scan types: a fixed scan time, a variable scan time, and
a variable scan time with upper limit.
6.2.1 Fixed Scan Time
When a fixed scan time is selected, the controller will attempt to start a new scan when the scan
time interval specified by the user expires. All tasks in the discrete scan will be executed once.
Then each task in the analog scan will be executed at least once.
