AutomationDirect DL105, User Manual

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RLL
P
LUS
Stage
Programming
7--15
RLL
PLUS
Stage Programming
DL105 PLC User Manual, 3rd Edition
Stage Program Design Considerations
The examples so far in this chapter used one self-contained state diagram to
represent the main process. However, we can have multiple processes
implemented in stages, all in the same ladder program. New stage programmers
sometimes try to turn a stage on and off each scan, based on the false assumption
that only one stage can be on at a time. For ladder rungs that you want to execute
each scan, just put them in a stage that is always on.
The following figure shows a typical application. During operation, the primary
manufacturing activity Main Process, Powerup Initialization, E-Stop and Alarm
Monitoring, and Operator Interface are all running. At powerup, three initial stages
shown begin operation.
Agitate
Monitor
Idle Fill Rinse Spin
E-Stop and Alarm Monitoring
Main Process
Operator Interface
Control Recipe
Status
XXX
= ISG
Powerup
Powerup Initialization
In a typical application, the separate stage sequences above operate as follows:
S
Powerup Initialization -- This stage contains ladder rung tasks done
just once at powerup. Its last rung resets the stage, so this stage is only
active for one scan (or only as many scans that are required).
S
Main Process -- this stage sequence controls the heart of the process
or machine. One pass through the sequence represents one part cycle
of the machine, or one batch in the process.
S
E-Stop and Alarm Monitoring -- This stage is always active because it
is watching for errors that could indicate an alarm condition or require an
emergency stop. It is common for this stage to reset stages in the main
process or elsewhere, in order to initialize them after an error condition.
S
Operator Interface -- this is another task that must always be active
and ready to respond to an operator. It allows an operator interface to
change modes, etc. independently of the current main process step.
Although we have separate processes,
there can be coordination among them.
For example, in an error condition, the
Status Stage may want to automatically
switch the operator interface to the status
mode to show error information as shown
to the right. The monitor stage could set
the stage bit for Status and Reset the
stages Control and Recipe.
Monitor
E-Stop and
Alarm Monitoring
Operator Interface
Control Recipe
Status
Set
Stage Program
Organization