RLL PLUS
Stage Programming
7–5
RLL
PLUS
Stage Programming
Right now, you may be thinking “I don’t see the big advantage to Stage
Programming... in fact, the stage program is longer than the plain RLL program”.
Well, now is the time to exercise a bit of faith. As control problems grow in complexity,
stage programming quickly out-performs RLL in simplicity, program size, etc.
For example, consider the diagram below.
Notice how easy it is to correlate the OFF
and ON states of the state transition
diagram below to the stage program at the
right. Now, we challenge anyone to easily
identify the same states in the RLL
program on the previous page!
S1
X0
JMP
SG
S0
S0
X1
JMP
SG
S1
OUT
Y0
OFF State
ON State
SP1
OFF ON
X0
X1
At powerup and Program-to-Run Mode
transitions, the PLC always begins with all
normal stages (SG) off. So, the stage
programs shown so far have actually had
no way to get started (because rungs are
not scanned unless their stage is active).
Assume that we want to always begin in
the Off state (motor off), which is how the
RLL program works. The Initial Stage
(ISG) is defined to be active at powerup. In
the modified program to the right, we have
changed stage S0 to the ISG type. This
ensures the PLC will scan contact X0 after
powerup, because Stage S0 is active.
After powerup, an Initial Stage (ISG)
works just like any other stage!
We can change both programs so that the
motor is ON at powerup. In the RLL below,
we must add a first scan relay SP0,
latching C0 on. In the stage example to the
right, we simply make Stage S1 an initial
stage (ISG) instead of S0.
S1
X0
JMP
ISG
S0
S0
X1
JMP
SG
S1
OUT
Y0
Initial Stage
SP1
S1
X0
JMP
SG
S0
S0
X1
JMP
ISG
S1
OUT
Y0
Initial Stage
SP1
X1X0
OUT
C0
OUT
Y0
C0
First Scan
SP0
Powerup in OFF State
Powerup in ON State
Powerup in ON State
Let’s Compare
Initial Stages
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