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The Operating
Sequence
Reading Running Algorithm Values
86 Programming the HP E1415 for PID Control
4
1
OUTPUT
INPUT
UPDATE
output table
from SCP
variables &
sent to SCP
channels,
algorothms
channels
analog &
digital
Set by ALG:OUTPUT:DELay (if any)
Trigger Event
Figure 3-8. Sequence of Loop Operations
The HP E1415 has four major operating phases. Figure 3-8 shows these
phases. A trigger event starts the sequence:
1. (INPUT); the state of all digital inputs are captured and each analog
input channel that is linked to an algorithm variable is scanned.
2. (UPDATE); The update phase is a window of time made large
enough to process all variables and algorithm changes made after
INIT. Its width is specified by ALG:UPDATE:WINDOW. This
window is the only time variables and algorithms can be changed.
Variable and algorithm changes can actually be accepted during other
phases, but the changes don't take place until an ALG:UPDATE
command is received and the update phase begins. If no
ALG:UPDATE command is pending, the update phase is simply used
to accept variable and algorithm changes from the application
program (using ALG:SCAL, ALG:ARR, ALG:DEF). Data acquired
by external specialized measurement instruments can be sent to your
algorithms at this time.
3. (CALCULATE); all INPUT and UPDATE values have been made
available to the algorithm variables and each enabled algorithm is
executed. The results to be output from algorithms are stored in the
Output Channel Buffer.
4. (OUTPUT); each Output Channel Buffer value stored during
(CALCULATE) is sent to its assigned SCP channel. The start of the
OUTPUT phase relative to the Scan Trigger can be set with the SCPI
command ALG:OUTP:DELay.
The PIDB algorithm stores its most important working values into the
Current Value Table (CVT) each time it executes. Further, by changing the
2
3
EXECUTE ALGS
execute all enabled algorithms
4
1
OUTPUT
INPUT
output table
from SCP
sent to SCP
channels,
channels
analog &
digital
Trigger Event
Chapter 3
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