Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Fri, 24 Feb 2012 12:46:39 -0600

Hi John,

On 2012-02-24 John Dobbins wrote:
> 
> This works for me with the following caveat. If the number of points in
> the waveform becomes greater than the number of points when the MEDM
> screen began execution the display does not adjust. It handles reducing
> the number of points and then increasing back to the original number but
> if I want to exceed the number of points that was  in effect when the
> screen began execution I have to switch to Edit and then back to Execute.
> 
> Is this what I should expect?

I can believe that MEDM currently behaves like that, although it seems like 
that should be a bug.  I suspect what's happening is that the graph widget is 
using the initial value from the count channel in the size of its subscription 
to the waveform data channel.  Closing and reopening the screen would delete 
and recreate those channels, cause the widget to read a new count value and 
thus subscribe for a larger number of data points.  This is what you're seeing 
happen.

I can see two conflicting uses for the count channel though and IMHO the 
second one makes it hard to change the existing behavior, although that 
doesn't mean we can't change the code so you can do what you want.

1. In your and Eric's case the count channel is being used to work around the 
problem that older versions of CA didn't support variable length arrays.  If 
MEDM were to subscribe for the waveform data giving a length of zero, recent 
versions of Base would only give it NORD elements automatically (older 
versions would always supply NELM elements although the values beyond NORD 
would all be zero).  I have asked Janet Anderson to look at how hard it would 
be to be able to configure the graph widget to do this.

2. The other case that I am concerned about is where the waveform might 
contain a large amount of valid data, and the count channel is being used to 
limit the graph to only subscribe for and display a small initial portion of 
the array.  If we were to modify MEDM to *not* use the count channel as a 
limit to the subscription size then we might break existing control systems as 
we could overwhelm the IOC's available bandwidth.

I think that adding the ability to tell the widget to use the new variable-
length array support should provide the functionality that you want without 
breaking any existing uses.  Hopefully Janet can find an easy way to add this 
feature and come out with a new release of MEDM that supports it (as long as 
you're using Base 3.14.12 for both your IOC and client, and you're not going 
through a gateway, since neither it nor the CAS support variable length arrays 
yet, unfortunately).

- Andrew
-- 
Never interrupt your enemy when he is making a mistake.
-- Napoleon Bonaparte

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System