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<== Date ==> <== Thread ==>

Subject: Re: synchronizing client requests & completions
From: "Peregrine M. McGehee" <[email protected]>
To: "Jeff Hill" <[email protected]>, "Alan K Biocca" <[email protected]>
Cc: "Marty Kraimer" <[email protected]>, "EPICS Techtalk" <[email protected]>
Date: Wed, 11 Mar 1998 16:52:20 -0000
Allan,
	You may be interested in the set of custom records that were originally
developed by the Gemini 8-m Telescopes Project for dealing with command
completion. Simple descriptions follow...
	
Command Action Directive (CAD)
The CAD record supports the essential features of a command including
validation of the input arguments, clearing and presetting of the inputs,
and distinguishing start and stop requests. The validation and argument
processing routine is supplied by the applications programmer. Although
many EPICS database records process when their inputs are written to, the
CAD record only processes when its trigger or directive field is updated,
thus ensuring that the command arguments constitute a synchronous and
consistent set. When a set of arguments have been successfully validated,
the supplied routine transfers that set (or values based on that set) to
the CAD record's outputs. The CAD record then activates the processing link
associated with the value of the directive to trigger other actions in the
database.

Command Action Response (CAR)
The CAR record holds the state of a command that has been accepted by the
database. This record is essentially a simple state machine that
transitions between IDLE, BUSY, ERROR, and PAUSED in response to inputs.

Apply
The Apply record synchronizes a collection of commands by driving the
processing of a set of CAD records in fixed order, and by accepting the
validation statuses of these CAD records as inputs. This record accepts CAD
record directives that are fanned out to the processed commands. The Apply
record also increments a client ID field each time the directive is
updated.

Aloha,
	Peregrine

Peregrine M. McGehee			Project Engineer: LEDA Control System
(505) 667-3273				[email protected]	
LANSCE-8 Controls & Automation	Los Alamos National Laboratory



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