AutomationDirect DL105, Руководство пользователя

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RLL
P
LUS
Stage
Programming
7--22
RLL
PLUS
Stage Programming
DL105 PLC User Manual, 3rd Edition
Q. Can I have a stage which is active for only one scan?
A. Yes, but this is not the intended use for a stage. Instead, just make a ladder rung
active for 1 scan by including a stage Jump instruction at the bottom of the rung.
Then the ladder will execute on the last scan before its stage jumps to a new one.
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:
S
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.
S
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.
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:
S
Use a stage JMP instruction for a state transition... moving from one
state to another.
S
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.
S
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.
Q. What is an initial stage, and when do I use it?
A. An initial stage (ISG) is automatically active at powerup. 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 have 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.