Control Usage Strategy

Lex Johnson’s rule of thumb about control block usage was posted to the mailing list in May 2007.

1. Do it in regulatory control blocks (most efficient, least prone to problems).

2. If the regulatory control blocks can’t do it, use a CALC block (efficient, but must deal with BAD and coding errors; debugging is a pain).

3. If the CALC block can’t do it, use a sequence block (pretty inefficient, but in the CP, so it’s redundant; code/compilation relatively complex; debugging is a pain).

4. If you can’t do it in a sequence block, reconsider if you need to do it.

5. If you must do it, see if you can do it in a shell (sh, ksh, csh, perl, etc.) script (easier to debug, easy to schedule, remember to use omgetimpand omsetimp to avoid broadcasts, remember to use show_params to check the IMPORT table).

6. If you can’t do it in a script, reconsider doing it.

7. If you must do it, see if a third-party or Foxboro package will do it. (Ask on the mailing list.)

8. If you must do it and can’t find a package, write a program.

Addendum (5 Sept 2008):

If you are going to use HLBL, keep the following in mind:

  • 1 line of HLBL code is roughly the same load as an AIN block.
  • External OM variable references, e.g., :C:B.P when the compound is in a different CP, suspend the block for one full block period. Thus, 20 lines setting values will take 10s to execute if the PERIOD is 0.5s.
  • Internal block references don’t suspend the block, but they are slower than you might expect and will have an impact on your loading if you have enough of them.
  • Remember that code runs until it hits a WAIT, WAITUNTIL, External Reference, or BPCSTM is reached.
  • Remember that if you don’t loop you code back to the top (yes, with a GOTO), it will reach the end of the block and stop.
  • Remember that ACTIVE and MA must both be set to true for the block to run.

added to The Cassandra Project Wiki by Duc M. Do, 15 May 2007.

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