Experimental Physics and
Industrial Control System

Ralph Lange <[email protected]> · Sat, 14 Jul 2007 20:25:42 +0200

I did a similar thing using a small database:
When changing (setting) a certain setpoint PV, a seq record (connected
through a CP input link to that setpoint) will set the SCAN fields of a
fixed list of related readback PVs to the fastscan setting and
reset/restart a countdown timer (calcout record scanned every second
counting its value down). Any subsequent "turning the knob" will again
reset the counter and thus extend the countdown timer period.

When the calcout reaches zero, it fires another seq record, which resets
the SCAN fields to the slowscan values.
Three more records, less functionality, but doesn't need changes to the
record definition and code, and is more robust: Reference counting
always implies statefulness, which introduces a number of problems
(handling of IOC reboots, clients not deassigning properly etc.)

Doesn't work for attached meters, though. Only for getting fast
responses on readbacks that are related to a knob that someone is turning.
It's the flexibility that makes EPICS confusing. And wonderful.
Ralph

John Sinclair wrote:

Actually, when I ported the tandem controls to EPICS, I modified a
couple of the

standard records (ai, ao, mbbiDirect, mbboDirect) to be able to
manipulate the scan value inside

the record's own code. The remote I/O throughput was too low to have
all the records scan at

a rate fast enough to make I/O sufficiently responsive during beam
tuning. In the old system

(no network), several process variables could be assigned to knobs and
meters which would

cause them to be processed at a higher scan rate. So I used the same
scheme. I added a

fastscan and slowscan field to the database along with several fields
to manage the assignment

operation. When the PV was "assigned" (to a knob or meter, in this
case), an assignment reference

count was incremented. When deassigned, the count was decremented.
When the count became

non-zero or zero, the scan field got the value of fastscan or slowscan
respectively. In this case

I obviously needed the record to manage the scan value rather than
some external agent.
Ain't EPICS wonderful.
John Sinclair
Mark Rivers wrote:

I think it would be a bad idea for a record to change the SCAN field.
That is something that you should let the database designer do.  They
may want to use your record in ways you never thought of.
However, if you really want your record to process again when something
happens then the record code can request reprocessing without changing
the SCAN field.  There are several ways to do this (scanOnce,
processRequestProcessCallback) depending on the application.  Look at
the Application Developer's Guide.
Mark

-----Original Message-----

From: [email protected]
[mailto:[email protected]] On Behalf Of Heinrich du Toit

Sent: Thursday, July 12, 2007 9:05 AM

To: TechTalk EPICS

Subject: Scan question
Can I change the SCAN field at runtime?
E.g. inside my own custom record.
Some devices normally needs only be scanned say once a sec or every 5
secs. But then when you change the value you want the scanned faster at
say 0.2 secs.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System