RLL PLUS
Stage Programming
7–6
RLL
PLUS
Stage Programming
We can mark our desired powerup state
as shown to the right, which helps us
remember to use the appropriate Initial
Stages when creating a stage program. It
is permissible to have as many initial
stages as the process requires.
OFF ON
X0
X1
Powerup
You may recall that a stage is just a section of ladder program which is either active or
inactive at a given moment. All stage bits (S0 to S377) reside in the PLC’s image
register as individual status bits. Each stage bit is either a boolean 0 or 1 at any time.
Program execution always reads ladder rungs from top to bottom, and from left to
right. The drawing below shows the effect of stage bit status. The ladder rungs below
the stage instruction continuing until the next stage instruction or the end of program
belong to stage 0. Its equivalent operation is shown on the right. When S0 is true, the
two rungs have power flow.
S If Stage bit S0 = 0, its ladder rungs are not scanned (executed).
S If Stage bit S0 = 1, its ladder rungs are scanned (executed).
SG
S0
Actual Program Appearance Functionally Equivalent Ladder
S0
(includes all rungs in stage)
The inline stage boxes on the left power
rail divide the ladder program rungs into
stages. Some stage rules are:
S Execution – Only logic in active
stages are executed on any scan.
S Transitions – Stage transition
instructions take effect on the next
occurrence of the stages involved.
S Octal numbering – Stages are
numbered in octal, like I/O points,
etc. So “S8” is not valid.
S Total Stages – The DL05 offers up
to 256 stages (S0 to S377 in octal).
S No duplicates – Each stage number
is unique and can be used just once.
S Any order – You can skip numbers
and sequence the stage numbers in
any order.
S Last Stage – the last stage in the
ladder program includes all rungs
from its stage box until the end coil.
SG
S0
SG
S1
SG
S2
END
What Stage Bits Do
Stage Instruction
Characteristics
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