[foxboro] IND block execution was: Setting Application Objects f rom IND blocks

Update to what I wrote before. More stuff for my book!!! :)

Regards,
 
Alex Johnson
Invensys Process Systems
Invensys Systems, Inc.
10707 Haddington
Houston, TX 77043
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713.722.2700 (switchboard)
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ajohnson@xxxxxxxxxxx


While on the topic of IND block execution:

1) It runs all code to completion when the block is Auto and Active.
2) The BPCSTM parameter will limit the number of statements per block
   period. 
3) A WAIT or WAITUNTIL will suspend the block's execution. It will check 
   the wait every BPC until it is time to move on. So a WAIT is not 
   completely free.
4) External references like yours (:A:O.A := 2) will suspend the block's
   execution. 
5) Internal references won't suspend the block's execution, but they are
   slow compared to other lines of code.
6) If the code terminates its execution, the block will go INACTIVE.
7) If the code loops without wait statements, the block will execute until
   the BPCSTM is reached and consume large percentages of the CP's CPU time.

On point 2, it is common to run sequence blocks at the BPC to simplify
scheduling and I suspect the writers of the code assumed that type of
scheduling when the created the name BPCSTM.

On point 4, there are several important things to understand:
   a) Ten external references in a block with a 0.5 block period will
      require 5 seconds to complete.
   b) Only one outstanding external reference is processed at a time.
      Additional references are queued. Thus, a bad reference can slow
      external references in other blocks. Consider the following example
      of a CP with two blocks in it each running at the BPC. In this
      case, the block processing would be something line:

      1) BPC1
         a) IND1 
              i) Makes an external reference to a missing point
             ii) The request is queued for processing
            iii) The block suspends.
         b) IND2 
              i) Make an external reference 
             ii) The request is queued
            iii) The block suspends
      
      2) BPC2
         a) IND1
              i) Wakes up
             ii) Checks for value
            iii) Value not available
             iv) Suspends    
         b) IND2
              i) Wakes up
             ii) Checks for value
            iii) Value not available
             iv) Suspends
      
      and so on and so on until the first request clears.
   
On points 6 and 7, the gist of the matter is that one should have all paths
lead to a WAIT and follow the wait with a GOTO <<START>> or something
similar if you want the block to stay active and not run amok while doing
so.
 
 
 
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