PLC scan cycle and process image
Цель: Изучить тему «PLC scan cycle and process image» и закрепить её практическими заданиями и самопроверкой.
2 акад. часа чтение и разбор теории, решение задач, работа со схемой или виртуальным стендом и проверка понимания.
Основной материал
Типовой цикл выполнения PLC
Classic PLC cycle:
Read physical inputs into process image.
Execute user program using snapshot.
Write output image to physical outputs.
Perform communications/diagnostics/system tasks.
This means input can physically change during scan, but program may not see change until next input update. Some systems allow direct peripheral access or event interrupts.
Scan time
Scan depends program size, instructions, communication and CPU speed. Infinite loop or excessive computation can trigger watchdog.
Race assumptions
Program is sequential, but engineer must understand order. If same output written in two places, last executed write may win. Such design is hard to maintain.
Практическая работа
Given a 20 ms scan, estimate maximum additional detection delay for an input changing just after scan copy (about one scan, plus output update). Discuss why safety response needs specified architecture.
Самопроверка
• What is process image?
• When are inputs read?
• What determines scan time?
• What is watchdog?
• Why multiple writes dangerous?
Что нужно запомнить
- Типовой цикл выполнения PLC Classic PLC cycle: Read physical inputs into process image.
- This means input can physically change during scan, but program may not see change until next input update.
- Some systems allow direct peripheral access or event interrupts.
- Scan depends program size, instructions, communication and CPU speed.
- Infinite loop or excessive computation can trigger watchdog.