Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Tue, 14 Aug 2007 14:36:38 +0200

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

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System