Experimental Physics and
Industrial Control System

Ben Franksen via Core-talk <[email protected]> · Thu, 24 Oct 2019 13:10:30 -0000

Below is a diff against commit f3204d8afad49b6f3aa375f7efe705e3b2f68229
for the doc fix.

We just checked that the behavior in 3.15. and 7.0 is the same for all
records that have DOL and OMSL: if DOL is constant, OMSL="closed_loop"
is ignored and the behavior is as if OMSL="supervisory". And modifying a
constant link with a CA put has no effect other than that the value of
the DOL field changes.

Your comment regarding the many changes to link support prompted me to
look at the new dbLink.h. It looks as if the existing anomalous behavior
of constant links is now hard-coded into the design of the LSET
interface. This makes it indeed more difficult to implement the change I
sketched above.

diff --git a/modules/database/src/std/rec/boRecord.dbd.pod b/modules/database/src/std/rec/boRecord.dbd.pod
index 8df8d7f..6b6d180 100644
--- a/modules/database/src/std/rec/boRecord.dbd.pod
+++ b/modules/database/src/std/rec/boRecord.dbd.pod
@@ -65,9 +65,9 @@ C<losed_loop> or C<supervisory>. If C<supervisory> is specified, the value
 in the VAL field can be set externally via dbPuts at run-time. If
 C<closed_loop> is specified, the VAL field's value is obtained from the
 address specified in the desired output location (DOL) field which can be a
-database link, a channel access link, or a constant. To achieve continuous
-control, a database link to a control algorithm record should be entered in
-the DOL field.
+database link or a channel access link, but not a constant. To achieve
+continuous control, a database link to a control algorithm record should be
+entered in the DOL field.
 
 L<Address Specification> presents more information on database addresses
 and links. L<Scanning Specification> explaines the effect of database

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https://bugs.launchpad.net/bugs/1844537

Title:
  OMSL="closed_loop" doesn't work with constant DOL

Status in EPICS Base:
  New

Bug description:
  This was incorrectly documented for the boRecord. I just fixed that in
  the wiki.

  However, I see this as yet another arbitrary limitation.

  To lift that we need a small generalisation to how we treat constant
  links. A constant link just has to remember whether it has been
  processed before. The flag gets set by dbGetLink and the like and it
  gets reset after link initialisation (so we can change the "constant"
  using a CA put to the link field). If dbGetLink finds that the link is
  constant, then it converts the value from the string stored in the
  link, but only if it hasn't been processed before. This also means
  that we can deprecate recGblInitConstantLink because it is now
  subsumed by dbGetLink.

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System