On Tuesday 14 August 2007 12:59, Heinrich du Toit wrote:
> What is the relationship between:
> a record
> a pv (process variable)
> and a record field
A PV is something a CA (Channel Access) client can try to connect to and
then communicate with. Thus it is the atomic unit of (remote)
communication. A CA server knows about a certain number of PVs and ansers
requests to (only) these PVs.
An EPICS IOC contains a CA server; however, it does not serve the PVs as a
flat list. Instead they are structured using records. Most record fields
are accessible, at least for reading, as a PV. Details of when and if a
record field is accessable depend on the record type and CA Security
configuration. For instance, field with type DBF_NOACCESS are not
accessible.
> Plus - Why does so many of the docs/presentations has so much on about
> channel access? I know it's important to epics internally... but for the
> most part it actually invisible to the "user" ?
The protocol itself is normally invisible (i.e. as long as everything works
as planned). However, some of CA's concepts need to be understood, mainly
the idea of a process variable, which basic operations are supported for
them (name lookup/connect, get, put, monitor). Also users should be able to
help themselves with low-level command line tools like cainfo, caget, caput
etc.
Cheers
Ben
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