Experimental Physics and
Industrial Control System

[email protected] · Tue, 1 Feb 2000 11:49:03 +0100 (MET)

  > [...] read the global variable at record initialization time [...]

  >   Comments?

Sure:

As low level inputs should IMHO always reflect the status of the
underlying devices (or structures) as close as possible, I see no
problem reading the variable value at init time.

Nevertheless, the variable support is so generic that you could think of
a situation where you don't want the record to read that variable until
a certain point, which may be well after record initialization. (Maybe
the first processing being triggered by the underlying task after it
wrote the variable for the first time.)

The PINI field could handle these two cases cases for input records.

For an output, I can think of three reasonable choices that may be
controlled without having to introduce another field in OUT:

1. PINI = NO, DOL = don't care
   If DOL is constant, set VAL to it, but don't do anything else . The
   underlying application takes care of all initialisation issues. When
   the value is set from the database or CA, write the variable.

2. PINI = YES, DOL is set (constant or link)
   Write the constant (OMSL being supervisory) or DOL-fetched (OMSL
   being closed_loop) value to the variable.

3. PINI = YES, DOL is not set
   Read back the variable value at startup. (I.e. the application set it
   to some meaningful value before record initialisation.)

The only problem could be that the default value for DOL changed between
some betaSomething revisions of 3.13.0 and older versions cannot distinct
between DOL not being set and being set to CONSTANT 0.

Some of the database configuration tools (i.e. CapFast) do not handle
the new situation correctly. Users may have problems as they might not
be able to _not_ set a DOL field. (CapFast drawings tend to set all
record fields even if the values are default. The CapFast conversion
tool configuration has to be changed to reflect the dbStatic changes.
Still there might be a difference between setting a record field to the
empty string and not setting it at all.)

What do you think?

Ralph

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System