1. Lecture Number 3
PLC Scanning

2. In this session…
In this session, you will learn how the PLC processes information.

3. Learning outcomes At the end of this session you should be able to:
Understand the PLC scanning process
Understand how different programming methods are affected by scanning
Understand how scanning affects processing speed

4. Introduction to Programmable Logic Controllers Scanning
By Clifford Mayhew

5. Architecture and Scanning

6. Architecture and Scanning

7. Architecture and Scanning Effects on Programming
Because the I/O is not scanned during programme execution, an input be activate and then de-activate within one programme scan.
This is called a Biphase Error.
High speed inputs can be handled using specialised high speed counter modules of interrupt inputs.

8. Architecture and Scanning Effects on Programming

9. Architecture and Scanning Effects on Programming

10. Architecture and Scanning
Exercise
The execution times for a processor are as follows:
Digital I/O 3.5ms
Overhead Update 100ms
Draw a timing diagram for the program below indicating the maximum time it will take O4 to energise after I1 has been activated.
Carry out the same exercise if the rung order is switched. I1 O1 O2 O3 O4

11. Summary
You should now have a good understanding of scanning and its effects on systems and cycle speed.