Experimental Physics and
Industrial Control System

Ralph Lange <[email protected]> · Mon, 07 Feb 2005 13:22:04 +0100

Didn't want to cause confusion ... sorry!
The PV name length restrictions within Channel Access have been gone for
quite a while. (Actually: years, I guess.) The only remaining limit is
the UDP packet size. (Hope you never have to type _that_ name.)
Keep in mind that things are different for strings within Channel Access
data - as these are bound to the predefined DBR_* types they are
restricted to the DBR_* type and length limits.
Cheers,
Ralph

Andy Foster wrote:
Hi Ralph,
I'm confused now. I though the client tool limit for searching for
PV names was set by the maximum string length in Channel Access
i.e. 40 characters, which hasn't changed right?
So, when the maximum PV name length was 29 chars in base, since this was
less than 40, it was all OK. What happens now when you have a PV
name longer than 40 chars, does the CA search call just truncate
the name? Or have I completely misunderstood the way the search
name is passed over CA?
Cheers,
Andy

On Mon, 7 Feb 2005, Ralph Lange wrote:

It depends.
The length of the records' name fields has been discussed a lot in the
past - after some intermediate steps it has been raised for EPICS Base
3.14 to its current value of 61 bytes (which is 60 characters plus the
NULL). (Any record's include file in <BASE>/include will have the
definite answer in such cases - for any version of Base.)
Note that this is the limit of the EPICS IOC database record names. A PV
name (e.g. for Channel Access) may be longer (else you wouldn't be able
to address fields of a record with a 60 char name).
You are correct that other tools might have different limits. The chance
of running against a hardcoded limit of 28 or 40 characters actually
increases with the tool's age - EDM most probably being a safe choice.
Luckily enough EPICS stuff is distributed as source code so you at least
have a chance to fix things you don't like. ;-)
Good luck,
Ralph

Gavin Smith wrote:

Hello All,
What is the character limit on the length of PV names in EPICS? I have
seen things referring to 28 chars, 40 chars and 60 chars. Whilst I
have sucessfully made a record with a double alphabet name (i.e.
26x2=52 chars) I am wary that other programs (such as EDM) may have
different limits.
Thanks,
Gavin Smith.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System