Chapter 2 - The Programmable Logic Controller
from one I/O update to the next is referred to as Scan Time. Scan time is typically measured in milliseconds and is related to the speed of the CPU and the length of the ladder diagram that has to be solved. The slower the processor or the longer the ladder diagram, the longer the scan time of the system. The speed at which a PLC scans memory is referred to as Scan Rate. Scan rate units are usually listed in msec/K of memory being utilized for the program. As an example, if a particular PLC has a rated scan rate of
8 msec/K and the program occupies 6K of memory, it will take the PLC 48 msec to complete one scan of the program.
2-9. Summary
Before a study of PLC programming can begin, it is important to gain a fundamental understanding of the various types of PLCs available, the advantages and disadvantages of each, and the way in which a PLC executes a program. The open frame, shoebox, and modular PLCs are each best suited to specific types of applications based on the environmental conditions, number of inputs and outputs, ease of expansion, and method of entering and monitoring the program. Additionally, programming requires a prior knowledge of the manner in which a PLC receives input information, executes a program, and sends output information. With this information, we are now prepared to begin a study of PLC programming techniques.
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Chapter 2 - The Programmable Logic Controller
Chapter 2 Review Questions
1.How were early machines controlled before PLC's were developed?
2.When were the first PLC's developed?
3.What is a shoe box PLC?
4.List four types of I/O modules?
5.List five devices that would be typical inputs to a PLC. List five devices that a PLC might control.
6.What types of memory might a PLC contain?
7.Which type or types of memory would store the program to be executed by the PLC?
8.What is the purpose of the programming unit?.
9.What type of control system did the PLC replace? Why was the PLC better?
10.What industry was primarily responsible for PLC development?
11.What are the two steps the PLC must perform during operation?
12.Describe I/O Update.
13.What is the Output Image Register?
14.Describe the procedure for solving a rung of logic.
15.What are the allowed direction of current flow in a ladder logic rung?
16.Define scan rate.
17.If a PLC program is 7.5K long and the scan rate of the machine is 7.5 msec/K, what will the length of time between I/O updates be?
18.Define scan time.
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Chapter 2 - The Programmable Logic Controller
19.At what time is data transferred to and from the outside world into a PLC system?
20.What common devices may be used to understand the operation of coils and contacts in ladder logic?
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