Mitsubishi Electronics FX1S manual Initial Steps, Terminating an STL Program

FX Series Programmable Controllers STL Programming 3

Initial Steps

For an STL program which is to be activated on the initial power up of the PLC, a trigger similar to that shown opposite could be used, i.e. using M8002 to drive the setting of the initial state.

The STL step started in this manner is often referred to as the initial step. Similarly, the step activated first for any STL sequence is also called the initial step.

M8002	SET	S005
STL	X001	Y000
S005
	X000
		Y011
	X012	Y014
	X013

3.3.3Terminating an STL Program

Once an STL program has been started the programmable controllers CPU will process all fol- lowing instructions as being part of that STL program. This means that when a second pro- gram scan is started the normal instructions at the beginning of the program are considered to be within the STL program. This is obviously incorrect and the CPU will proceed to identify a programming error and disable the programmable controllers operation.

This scenario may seem a little strange but it does make sense when it is considered that the STL program must return control to the ladder program after STL operation is complete. This means the last step in an STL program needs to be identified in some way.

Returning to Standard Ladder

This is achieved by placing a RET or RETurn instruction as the last instruction in the last STL step of an STL program block.

This instruction then returns programming con- trol to the ladder sequence.

M8002	SET	S005
STL	X001	Y000
S005
	X000
		Y011
	X012	Y014
	X013
		RET

Note: The RET instruction can be used to separate STL programs into sections, with stan- dard ladder between each STL program. For display of STL in SFC style format the RET instruction is used to indicate the end of a complete STL program.

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