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Programmable Logic
Controllers
Interrupt Function Files:
Selectable Timed Interrupt (STI)
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
Selectable Timed Interrupt (STI)
Interrupt Function Files
• There are two basic types of interrupt
function files
- Selectable Timed Interrupt (STI) Function
File
- For periodic function execution
- Event Input Interrupt (EII) Function File
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
- For function execution based on an event
Using the Selectable Timed Interrupt
(STI) Function File
• The Selectable Timed Interrupt (STI)
provides a mechanism to solve time critical
control requirements.
• The STI is a trigger mechanism that allows
you to scan or solve control program logic
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
y p g g
that is time sensitive.
STI Example Usage
• Examples of where you would use the STI
include
- PID type applications, where a calculation must be
performed at a specific time interval.
- A motion application, where the motion instruction
(PTO) needs to be scanned at a specific rate to
guarantee a consistent acceleration/ deceleration
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
g /
profile.
- A block of logic that needs to be scanned more
often.
STI Function File Sub-Elements
Summary
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
STI Program File Number (PFN)
- The PFN (Program File Number) variable defines which subroutine is called (executed) when the timed interrupt times out. - A valid subroutine file is any program file (3 to 255)
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
A valid subroutine file is any program file (3 to 255).
- The subroutine file identified in the PFN variable is not a special file within the controller; it is programmed and operates the same as any other program file.
- From the control program perspective it is unique, in that it is automatically scanned based on the STI set point.
STI Error Code (ER)
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
- Error codes detected by the STI sub-system are
displayed in this register.
STI User Interrupt Executing (UIX)
- The UIX (User Interrupt Executing) bit is set whenever the STI mechanism completes timing and the controller is scanning the STI PFN Th UIX bi i l d h h ll l
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
STI PFN. The UIX bit is cleared when the controller completes processing the STI subroutine.
- The STI UIX bit can be used in the control program as conditional logic to detect if an STI interrupt is executing.
STI User Interrupt Pending (UIP)
- The UIP (User Interrupt Pending) is a status flag that represents an interrupt is pending. This stat s bit can be monito ed o sed fo logic p poses in the
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
- This status bit can be monitored or used for logic purposes in the control program if you need to determine when a subroutine cannot execute immediately.
- This bit is automatically set and cleared by the controller. The controller can process 1 active and maintain up to 2 pending user interrupt conditions before it sets the lost bit.
STI Timed Interrupt Enabled (TIE)
- The TIE (Timed Interrupt Enabled) control bit is used to enable or disable the timed interrupt mechanism. When set (1) timing is enabled hen clea (0) timing is disabled
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
- When set (1), timing is enabled, when clear (0) timing is disabled.
- If this bit is cleared (disabled) while the timer is running, the accumulated value is cleared (0). If the bit is then set (1), timing starts.
- This bit is controlled by the user program and retains its value through a power cycle.
STI Auto Start (AS)
- The AS (Auto Start) is a control bit that can be used in the control program.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
- The auto start bit is configured with the programming device and stored as part of the user program.
- The auto start bit automatically sets the STI Timed Interrupt Enable (TIE) bit when the controller enters any executing mode.
STI Error Detected (ED)
- The ED (Error Detected) flag is a status bit that can be used by the control program to detect if an error is present in the STI sub-system.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 14-
sub syste
- The most common type of error that this bit represents is a configuration error.
- When this bit is set, the user should look at the error code in parameter STI:0.ER
- This bit is automatically set and cleared by the controller.
