
B–61863E/15
2. STEP SEQUENCE BASICSSTEP SEQUENCE FUNCTION
977
(3) Transition
Pn
A transition denotes the transition conditions. When these evaluate
true, the step of the corresponding state changes from the inactive to
active state or vice the reverse. Specify the P address (Pn), which
indicates a subprogram describing the transition conditions in detail.
As shown in Fig. 2.1 (c), step S2 changes its state from inactive to
active when the conditions described in transition P10 evaluate true,
while step S2 changes its state from active to inactive when the
conditions described in transition P20 evaluate true.
Executing step1 Executing step2 Executing step3
S1 (step 1) S1 (step1) S1 (step1)
P10 (Condition
is true)
S2 (step2)
P20
P20 (Condition
is true)
S3 (step3)
P10
S2 (step2)
S3 (step3)
P20
P10
S2 (step2)
S3 (step3)
Fig. 2.1 (c) Transition of step state by the transition
Note that the step immediately before a transition must be active in
order to switch the next step from inactive to active when the
conditions specified in the transition evaluate true. As shown in
Fig. 2.1 (d), step S3 does not change to the active state, even when
transition P20 evaluates true, if step S1 is active and step S2 is
inactive. An active state passes from a certain step to the next step
when the corresponding transition conditions evaluate true, the
execution of the step sequence program advancing one step.
Executing step1 Executing step1
S1 (step1) S1 (step1)
P10
S2 (step2)
P20 (Condition
is true)
P20
S3 (step3)
P10
S2 (step2)
S3 (step3)
Fig. 2.1 (d) Transition of step state by transition