Re: histogram record device support

[Andrew Johnson <anj@aps.anl.gov>]

Graham Waters wrote:

> I am trying to add VME_IO support to the histogram record, specifically
> the Highland V680 Time-To-digital Converter. However try as a might
> and dispite anything I place in my .dbd file eg:
>    device(histogram, VME_IO, devHiV680, "V680")
> the SVL field in my database is always over-written with "PV_LINK" or
> "CA_LINK".

Hmm, interesting.  You must be the first person to try writing hardware 
device support for the histogram record since R3.11.x or even earlier, and 
it appears to me that nobody else can currently be using anything other 
than soft support with it.  Here's why:

The static database access library used by iocCore maintains an implied 
connection between the DTYP field of a record and one other link field 
which must be named either "INP" or "OUT".  The setting in the DTYP field 
determines what the link type is of the related field; all other link 
fields have their types set to be PV links (or constants).

The histogram record doesn't have either an INP of OUT field, it appears 
to use the field name "SVL" instead.  Therefor it is currently impossible 
for the link type of that field to be set to anything other than a PV 
link, which will be converted to either a DB or CA link at runtime.

There are three ways to fix this, in order of increasing preference:

1. Rename the SVL field to INP would seem to be the logical change, but 
would break all databases that currently set the SVL link.  I don't think 
that would be acceptable to existing sites.

2. Add SVL to the list of fields that dbStaticLib associates with the DTYP 
field of a record.  This would be fairly simple to do (at a first glance I 
can only find 5 places in dbStaticLib.c that need a one-line addition), 
but it's a pity to have to add another exception just for one record type 
(which should really have used the INP name).

3. Change your device support so that it expects a CONSTANT in the SVL 
field, which used as an index into a table of V680 cards that have been 
registered with it in the st.cmd file before iocInit().  This approach 
will get you going without making any changes to Base or existing databases.


> Also what field points to the histogram array so I can plot it with DM

It looks like the VAL field should provide access to the array (it is 
marked SPC_DBADDR, and the cvt_dbaddr() routine looks to be setting the 
right kind of stuff for this), but I'm not an experienced user of the 
histogram record so I could be wrong.

- Andrew
--
"Life is what happens while you're busy making other plans." - John Lennon