Automationdirect.com D4-454 PLC User Manual page 408

Chapter 7: RLLPLUS Stage Programming
Q. Isn't a Stage JMP just like a regular GOTO instruction used in
software?
A. No, it is very different. A GOTO instruction sends the program execution immediately
to the code location named by the GOTO. A Stage JMP simply resets the Stage Bit of
the current stage, while setting the Stage Bit of the stage named in the JMP instruction.
Stage bits are 0 or 1, determining the inactive/active status of the corresponding stages.
A stage JMP has the following results:
Q. How can I know when to use stage JMP, versus a Set Stage Bit or
Reset Stage Bit?
A. These instructions are used according to the state diagram topology you have derived:
Q. What is an initial stage, and when do I use it?
A. An initial stage (ISG) is automatically active at power up. Afterwards, it works just like
any other stage. You can have multiple initial stages, if required. Use an initial stage for
ladder that must always be active, or as a starting point.
Q. Can I place program ladder rungs outside of the stages, so they are
always on?
A. It is possible, but it's not good software design practice. Place ladder that must always
be active in an initial stage, and do not reset that stage or use a Stage JMP instruction
inside it. It can start other stage sequences at the proper time by setting the appropriate
Stage Bit(s).
Q. Can I have more than one active stage at a time?
A. Yes, and this is a normal occurrence for many programs. However, it is important to
organize your application into separate processes, each made up of stages. And a good
process design will be mostly sequential, with only one stage on at a time. However, all
the processes in the program may be active simultaneously.
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D4-454 PLC User Manual, 1st Edition, Rev. D
• When the JMP is executed, the remainder of the current stage's rungs are
executed, even if they reside past(under) the JMP instruction. On the following
scan, that stage is not executed, because it is inactive.
• The Stage named in the Stage JMP instruction will be executed upon its next
occurrence. If located past (under) the current stage, it will be executed on the
same scan. If located before (above) the current stage, it will be executed on the
following scan.
• Use a Stage JMP instruction for a state transition ... moving from one state to
another.
• Use a Set Stage Bit instruction when the current state is spawning a new parallel
state or stage sequence, or when a supervisory state is starting a state sequence
under its command.
• Use a Reset Bit instruction when the current state is the last state in a sequence
and its task is complete, or when a supervisory state is ending a state sequence
under its command.