ABB ST语言编程(9)

System 800xA Training

IFs may be nested to many levels. A better structure to be used instead of nesting isthe CASE structure.This is very useful when many conditions need evaluation; thereare several forms:CASE…OF

Operator

CASE Integer Expression OF

Integer Literal1 : Statement(s);

END_CASE;value of an integer variable or an integerexpression.

The <integer literal> is one or several integer

values or one or several ranges of values.

the appropriate statements) executed. One

variation is to permit a range of values to be

tested rather than an exact single value.

If none of the test literals match the result of

the expression then the statement(s) in the

ELSE clause will be executed. If no ELSE

exists, none of the statements will be

executed.CASE Integer Expression OFInteger Literal1 : Statement(s);Integer Literal2 : Statement(s);Integer Literal3: Statement(s);ELSEStatement(s);END_CASE;

13.3.6Iteration Structures

Several iteration constructs exist: They should be used with extreme caution. Duringthe iteration of the loop, the CPU remains in the loop. If a loop has a large number ofiterations then the normal scan time may be easily exceeded.

NOTE!Only use these constructs when you know exactly howmany iterations are going to be done.iption

statements) to be executed repeatedly for a given

number of times. The counter used in the repetition

process can be used in the statements.

In the example, the statements between the FOR

and END_FOR will be executed 16 times.

WHILELevel > 80.0 DO

Statement(s);

END_WHILE;WHILE statement is used in order to allow astatement (or statements) to be executed repeatedlywhile a certain condition is True.

This separates it from theFOR statement. It has

some similarities with theREPEAT statement.

REPEAT statement is used in order to allow a

statement (or statements) to be executed repeatedly

until a certain condition is True.

Note that the test to exit the loop is placed at the end,

so a minimum of one execution of the statement(s)

will occur even if the expression is true at the time

the loop is entered.

EXIT statement whenever you want to

terminate a loop immediately and continue execution

from the first line after the iteration statement.OperatorFORi := 0 to 15 DOStatement(s);END_FOR;REPEATStatement(s);UNTILBoolean ExpressionEND_REPEAT;EXIT

Chapter13-9