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5-21 Step Instructions
The step instructions STEP(08) and SNXT(09) are used in conjunction to set
up breakpoints between sections in a large program so that the sections can
be executed as units and reset upon completion. A section of program will
usually be defined to correspond to an actual process in the application. (Re-
fer to the application examples later in this section.) A step is like a normal
programming code, except that certain instructions (e.g. IL(02)/ILC(03),
JMP(04)/JME(05)) may not be included.
5-21-1 STEP DEFINE and STEP START–STEP(08)/SNXT(09)
B: Control bit
IR, AR, HR, LR
Ladder Symbols Definer Data Areas
STEP(08) B
STEP(08)
B: Control bit
IR, AR, HR, LR
SNXT(09) B
Limitations All control bits must be in the same word and must be consecutive.
STEP(08) uses a control bit in the IR or HR areas to define the beginning of
a section of the program called a step. STEP(08) does not require an execu-
tion condition, i.e., its execution is controlled through the control bit. To start
execution of the step, SNXT(09) is used with the same control bit as used for
STEP(08). If SNXT(09) is executed with an ON execution condition, the step
with the same control bit is executed. If the execution condition is OFF, the
step is not executed. The SNXT(09) instruction must be written into the pro-
gram so that it is executed before the program reaches the step it starts. It
can be used at different locations before the step to control the step accord-
ing to two different execution conditions (see example 2, below). Any step in
the program that has not been started with SNXT(09) will not be executed.
Once SNXT(09) is used in the program, step execution will continue until
STEP(08) is executed without a control bit. STEP(08) without a control bit
must be preceded by SNXT(09) with a dummy control bit. The dummy con-
trol bit may be any unused IR or HR bit. It cannot be a control bit used in a
STEP(08).
Execution of a step is completed either by execution of the next SNXT(09) or
by turning OFF the control bit for the step (see example 3 below). When the
step is completed, all of the IR and HR bits in the step are turned OFF and all
Description
Step Instructions Section 5-21
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