Mitsubishi Digital Electronics FX2NC Manual

A SERVICE OF

next previous

FX Series Programmable Controllers STL Programming 3

3-4

Initial Steps

For an STL pro

g

ram which is to be activated

on the initial power up of the PLC, a tri

gg

er

similar to that shown opposite could be used,

i.e. usin

g

M8002 to drive the settin

g

of the

initial state.

The STL step started in this manner is often

referred to as the initial step. Similarl

y

, the

step activated first for an

y

STL sequence is

also called the initial step.

3.3.3 Terminating an STL Program

Once an STL pro

g

ram has been started the pro

g

rammable controllers CPU will process all fol-

lowin

g

instructions as bein

g

part of that STL pro

g

ram. This means that when a second pro-

g

ram scan is started the normal instructions at the be

g

innin

g

of the pro

g

ram are considered to

be within the STL pro

g

ram. This is obviousl

y

incorrect and the CPU will proceed to identif

y

a

pro

g

rammin

g

error and disable the pro

g

rammable controllers operation.

This scenario ma

y

seem a little stran

g

e but it does make sense when it is considered that the

STL pro

g

ram must return control to the ladder pro

g

ram after STL operation is complete. This

means the last step in an STL pro

g

ram needs to be identified in some wa

y

.

Returning to Standard Ladder

This is achieved b

y

placin

g

a RET or RETurn

instruction as the last instruction in the last

STL step of an STL pro

g

ram block.

This instruction then returns pro

g

rammin

g

con-

trol to the ladder sequence.

STL

S005

M8002

SET S005

X001

X000

X012

X013

Y000

Y011

Y014

STL

S005

M8002

SET S005

X001

X000

X012

X013

Y000

Y011

Y014

RET

Note: The RET instruction can be used to separate STL pro

g

rams into sections, with stan-

dard ladder between each STL pro

g

ram. For displa

y

of STL in SFC st

y

le format the RET

instruction is used to indicate the end of a complete STL pro

g

ram.