Experimental Physics and
Industrial Control System

Steve Lewis <[email protected]> · Mon, 17 Oct 2005 10:41:06 -0700

Marty, thanks for your extremely insightful summary of 5

meetings...although I missed seeing all of you and everyone else in

person, I feel like you have pulled it together better than I would

if I had actually been there :-)

<RANT>

At 10:57 AM -0500 2005/10/17, Marty Kraimer wrote:

There are two things I want to emphasize.

1) EPICS has a narrow interface and ALAMA/ACS has a wide interface.

Narrow means that EPICS just has well defined data types and wide

means that ACS has object to object communication.

This is the key difference. The NIF control system also has an

ALMA_like, wide, object-to-object interface. My opinion is that this

nearly killed it. A wide interface is a very subtle way of replacing

the benefits of encapsulation and data hiding with (larger) problem

of "interface coupling".

You cannot effectively build tools; programmers need to be involved

at every level, forever. Even small changes require rebuilding

everything: it's "make clean; make" every night--hopefully you have a

full suite of regression tests with a lab filled with real hardware.

NIF has about $500K sunk in such a lab, and keeps its test team

staffed at 25% of the level of the developers.

DON'T DO THIS.

The following are the main features of EPICS IOCs

1) The IOC database is the heart of EPICS iocCore. It is what

distinguishes EPICS from the other control systems in use by the

accelerator and telescope control systems. Most of the others have

the concept of device servers in the front ends rather that an

intelligent runtime database, which means that the intelligence

resides in the workstations or else is implement in an ad-hoc way in

the device servers or in some other systems like PLCs.

YES, YES, YES. Repeat this every morning when you wake up; and every

evening before you go to sleep.

2) The interface to the IOC database is narrow in the sense that

communication is done via data rather than a large set of objects.

This is what allows general purpose tools like display managers and

what makes it easy for other systems to talk to IOCs. In EPICS

facilities it is common for operators to create displays for MEDM or

EDM. I suspect that tools like the the tango gui builder would be

used by programmers rather than operators or hardware oriented

engineers. Thus we must be carefull about how much device

orientation we introduce into IOCs. Perhaps a power supply but not a

high level physics abstraction.

Again, this is what the NIF architecture is like. It is (was)

extremely frustrating under this architecture to bring some

functionality to bear on the hardware and to the operators. What

would have taken 10 minutes with VDCT and 1 minute with MEDM in EPICS

took from 1 hour to 1 day from each of 1) the FEP programmer; 2) the

tier 2 programmer; 3) the IDL/SQL/RDB team; 4) the GUI programmer; 5)

the tier 3 team [sequencer-like "scripts"]; 6) the CM team to

"regenerate the IDL", do "make clean; make". And then you test.

Elapsed calendar time: weeks to months.

We called it "IDL hell" and employed the most devious strategies to

avoid items (2)-(6).

</RANT>

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System