Experimental Physics and
Industrial Control System

"Pete R. Jemian" <[email protected]> · Tue, 12 Dec 2000 15:54:34 -0600

Ignoring many other fine points raised,
At 10:08 PM 12-12-00 +0100, Ben Franksen wrote:

...
If CA is indeed to be revised, I want to use the opportunity and present
my idea how this should be done and how not. ...
...
(e) Restricting data type conversion to happen on the client side has a
lot of advantages: The client machines are usually more powerful than
IOCs. The IOC needs to perform real-time tasks, the clients usually
don't. Thus it makes sense to relieve the server of the task of
converting the data to the type desired by the client. Most existing
clients already request the native type in order to save bandwidth and
reduce IOC load.

This is a myopic point of view for the broad EPICS community.
Many clients (it may even be safe to say that most clients,
when one considers the clients most frequently connecting to PVs)
are IOCs with Channel-Access links and SNL programs.
Even without assuming that IOCs are the most common clients,
one recognizes that much server-side software relies on
processed information, rather than raw information.
One good example is the position of a motor.  The server
is the best place to locate the conversion (and various
constants) between motor (or encoder) pulses and position
in millimeters or degrees.  SNL programs that build abstract
coordinates from motor position are another example, such as
an X-ray monochromator.
It is unimaginable to attempt to convince our scientists in
the year 2001 to scan the X-ray monochromator from 12678766 to 9348754
theta motor pulses rather than 8.8 keV to 9.2 keV.  Dreadful.
Why should each and every client have to make these conversions?
What is a client/server system for?
Just because our servers are a generation old, (mutter, mutter, ...).
(f) Writing server tools will be simplified drastically (this is one of
the reasons I began to think about this).

Not simplified in many cases.
A major philosophical difference here, as follows:
When the programmer takes the easy way out, to simplify,
for whatever purpose, the work then falls to the user of
the software.  So, what was quite a bit of work for one
person now falls to many to repeat over and over.
It is the point of software to simplify complex tasks.
Otherwise, why bother with it at all?
Regards,
   Pete Jemian
   UNICAT

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System