Experimental Physics and
Industrial Control System

"Steven M. Hartman" <[email protected]> · Thu, 20 Jan 2011 09:47:36 -0500

Jimmy Johnson wrote:

 I am planning on using a SoftIOC (R3.14.12) on Linux and will want

to run a control loop (about 1000 lines of C code spread across a few

custom records) deterministically at 100Hz. There will not be any other

user applications, in addition to the IOC, running on the box.

How stringent is your "deterministically at 100Hz" requirement?

EPICS itself shouldn't be an impediment, but the operating

system's real time capabilities will be. I would not expect a

non-realtime OS to do particularly well, though. A quick test on a

lightly loaded desktop machine with a 2.6.18 kernel gives . . .

My100HzTest                    2011-01-20 09:20:36.881690 10
My100HzTest                    2011-01-20 09:20:36.892685 10.01
My100HzTest                    2011-01-20 09:20:36.903834 10.02
My100HzTest                    2011-01-20 09:20:36.914830 10.03
My100HzTest                    2011-01-20 09:20:36.925973 10.04
My100HzTest                    2011-01-20 09:20:36.937975 10.05
My100HzTest                    2011-01-20 09:20:36.950138 10.06
My100HzTest                    2011-01-20 09:20:36.962171 10.07
My100HzTest                    2011-01-20 09:20:36.974273 10.08
My100HzTest                    2011-01-20 09:20:36.986281 10.09
My100HzTest                    2011-01-20 09:20:36.997378 10.1
 . . .
My100HzTest                    2011-01-20 09:20:38.013803 11
My100HzTest                    2011-01-20 09:20:38.024939 11.01
 . . .

(a calc record incrementing by 0.01, with a custom SCAN of 0.01

seconds)

will show you that each iteration is going to happen slightly

later then ideal, in this case loosing 0.13 seconds over 100

iterations.

For a realtime OS like VxWorks, 100 Hz should be easy as long as

your processing can complete within that 0.01 second window

(dependent on your processor and your code and task priorities). I

don't have any experience with realtime extensions to Linux so

don't know how well that will perform, but if you are interested

in using Linux, I would start with that.

--
Steven Hartman
[email protected]

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System