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D := TEST(1.3, 3.4); (* sample calling program, here C will default to 3.14 *) E := TEST(1.3, 3.4, 6.28); (* here C will be given a new value *) ....

FUNCTION TEST : REAL VAR_INPUT A, B : REAL; C : REAL := 3.14159; END VAR TEST := (A + B) / C;

END_FUNCTION

Figure 19.23 Declaration of a Function

19.3 SUMMARY

plc st -19.20

1. PRACTICE PROBLEMS
2. PRACTICE PROBLEM SOLUTIONS
1. ASSIGNMENT PROBLEMS
2. 1. Write logic for a traffic light controller using structured text.
3. SEQUENTIAL FUNCTION CHARTS

plc sfc -20.1

Topics:

* Describing process control SFCs
* Conversion of SFCs to ladder logic

Objectives:

* Learn to recognize parallel control problems.
* Be able to develop SFCs for a process.
* Be able to convert SFCs to ladder logic.

20.1 INTRODUCTION

All of the previous methods are well suited to processes that have a single state active at any one time. This is adequate for simpler machines and processes, but more complex machines are designed perform simultaneous operations. This requires a controller that is capable of concurrent processing -this means more than one state will be active at any one time. This could be achieved with multiple state diagrams, or with more mature techniques such as Sequential Function Charts.

Sequential Function Charts (SFCs) are a graphical technique for writing concurrent control programs.

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