Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Thu, 19 Mar 2015 14:34:25 -0500

Hi Alex,

On 03/19/2015 12:30 PM, Alex Wallace wrote:
> It seems to me that the mbbi record should have a field for the default
> value when the input value is undefined. At least a field for the
> string, so HMIs can show that the record is in an error state. Severity
> is good, but without a string to give it meaning... thoughts?

We could add an UDST field to the mbbi/mbbo records to be used when the
VAL field is outside the range 0..15, but many CA clients request the
value of an ENUM field as an integer (and fetch the full list of strings
for each ENUM). Unfortunately the Channel Access protocol has no way to
tell clients what string to display when the ENUM value is outside that
range, so many clients such as MEDM would never show the contents of
that UDST field.

If you ask the IOC for the string representation of an ENUM value
outside the range you get back "Illegal Value", but a Text widget on an
MEDM screen just shows a blank. The caget program prints the string
"Illegal Value", but each CA client is likely to have its own response.

Adding a field for this would be a simple thing to do, but it only
solves a tiny part of the problem. To finish it we'd have to modify the
CA protocol, change the client API and several of the data types to
provide the new undefined_string value, update both the C++ and Java
implementations of the CA client and server code, and fix all the CA
client programs to understand the modified data types.

- Andrew
-- 
Light thinks it travels faster than anything but it is wrong.
No matter how fast light travels, it finds the darkness has
always got there first, and is waiting for it.
    -- Terry Pratchett, Reaper Man

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System