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<== Date ==> <== Thread ==>

Subject: Re: MBBO record support
From: Peregrine McGehee <[email protected]>
To: [email protected]
Date: Fri, 20 Oct 1995 08:05:41 -1000

Marty writes:
------- start of forwarded message (RFC 934 encapsulation) -------
> Item 1.
> We have noticed the following behavior in the MBBO record when we have
> a constant DOL field.
...
> The net effect is that VAL never has the value of DOL if DOL is a
> constant.
> 

I think that this is the exact same problem pointed out by William Lupton a
short time ago. There were a few E-mail messages (not many) that gave 
contradictory recommendations about what to do. 

This is a small part of the problem of what to do about outputs when an ioc
reboots. Here are some possibilities:

1) It an output device provides the ability to read the current value
   of the output, then the initial VAL of the record should be set to the
   ENG value associated with the raw value read from the device.

   THIS is the method used by most epics supplied device support.

2) If 1) is not possible do not write anything to the device until the record
   is processed the first time. Note that this may not always be easy.
   Normally an output device has many signals (ao have multiple channels,
   bo has may bits) that are all written simultaneously. These signals
   are shared by multiple records. When one record requests a write ALL the
   signals are written.

3) Accept initial value from user. In above message a CONSTANT link value
   was used to attempt this. One problem currently is that we cant distinguish
   between the user actually specifyiing a constant link value and the user
   not specifying any value for a link.
   Note that the message above shows that the record/device support did this
   UNLESS 1) was possible.

Can we even define a valid set of things to do? let me give an example.

If a true warm reboot ios being attempted, i.e. the system is operational
and running just fine, always do 1).
If the system is being cold started do 3)

How do we distinguish between these in an automatic mannor? We cant expect
a human to make these decisions on a record by record basis when a system
is booted.

> 
> Item 2.
> Bob Dalesio mentioned that a field known as "IVAL" used to exist on
> the MBBO record. The purpose of this field was to allow puts that were
> not of type DBR_ENUM into the VAL field thus circumventing the
> complaints from put_enum_string(). This is a useful feature if you
> have no state strings and associated values defined (SDEF=0).
> 

This was available in gtacs but never if epics. I dont remember the details
but it was somthing almost impossible to save in going to extensible
record/device/driver support, whicjh was the main feature epics provided that
gtacs did not.

Again I maintain that the main problem is using mbbi,mbbo records for holding
arbitrary integers.


Marty Kraimer
------- end -------


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