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Topic Details for Soft Logic

Program Control Instructions

Program control instructions control the flow of the PLC program.

Instr.	Descr.	# Param	Subr	DS	KInt	One Shot
END	Program end	0
ENDC	Program end conditional	0
FOR	For/next loop	1 or 2		X	X	X
NEXT	Next in For/next loop	0
SBR	Define a subroutine	1	X
CALL	Call subroutine	1	X
RT	Return from subroutine	0
RTC	Return from subroutine conditional	0

Program END

END and ENDC will terminate a program scan. END terminates the scan unconditionally, while ENDC will terminate the scan if the logic stack is true.

Example:


	// END.
	NETWORK 100
	STR SC1
	AND X5
	ENDC
	STRN SC1
	END

Subroutines

SBR, CALL, RT, and RTC are subroutine instructions. SBR is used to define a subroutine. CALL will call a subroutine. SBR and CALL expect a valid subroutine name as a parameter. RT will return from a subroutine unconditionally. RTC will return from a subroutine if the logic stack is true.

Example:


	// Subroutines.
	NETWORK 1
	STR SC1
	OUT Y1
	CALL SubTest
	
	NETWORK 2
	STR C47
	OUT Y19
	
	END
	
	// Define subroutine.
	SBR SubTest
	NETWORK 1
	STR SC1
	RST Y10
	AND X21
	RTC
	SET C47
	RT

For/Next

FOR and NEXT are used to define a loop which executes a specified number of times. FOR begins the loop, and NEXT defines the end of the loop. FOR expects an integer parameter specifying the number of loops to perform. This may be a register or constant. FOR also accepts an optional parameter specifying whether the instruction should execute as a one-shot. FOR/NEXT loops may be nested.

Example:


	// FOR/NEXT with constant.
	NETWORK 1
	STR SC1
	FOR 10
	OUT Y1
	NEXT
	
	// FOR/NEXT with register.
	NETWORK 2
	STR SC1
	FOR DS1
	OUT Y2
	NEXT
	
	// FOR/NEXT with one-shot.
	NETWORK 2
	STR SC1
	AND C1
	FOR 100 1
	OUT Y2
	NEXT

Errors

Using RT or RTC in the main program will cause a run-time error. Using NEXT without FOR may cause a run-time error. Using FOR without NEXT may cause unpredictable run-time operation.

Subroutines may call other subroutines, and they may even call themselves (recursion is permitted). However, if if the nesting level reaches an excessive level, a run time error will occur and the program will exit. The maximum nesting level is undefined, but is typically about 1000.

Ladder Examples

The following shows examples in ladder format. Each example shows the IL code as comments, followed by the ladder equivalent.

program control example 1 program control example 2 program control example 3 program control example 4