Experimental Physics and
Industrial Control System

[email protected] (Nick Rees) · Mon, 28 Nov 94 16:23:38 HST

I have a rotary encoder with a range of travel less than 360 degrees
that I want to read into an analog input record. (Yes, its the
telescope mount encoder).  Unfortunately I have no guarantee where the
encoder zero point is. The zero point adjustment has to be added to the
raw value in encoder units, and then the value circularly normalised so
the value from the encoder always lies between two accepted values.

I have to write analog input device support for the hardware, so I was
intending to do this fiddling in device support. Looking at the record
support, it would seem that the ROFF field might be a field I could use
- it is in digitizer units (the most convenient in this case) and is
added to the raw value just after it is returned from device support.
Device support would then:

a. Read the value.
b. Add ROFF
c. Circularly normalize between the maximum and minimum possible values.
d. Subtract ROFF (so that it can be added once again in record support).

A few questions:

1. How do I set ROFF? The documentation says `it is the responsibility
   of the device support routines to use EGUF and EGUL to compute ESLO
   and ROFF'. All very well but none of the device support routines I
   have reference ROFF. (grep roff ~epics/share/src/dev/*.c returns nothing).
   What gives? Am I missing a trick somewhere?

2. Can I use EGUF and EGUL in ways that were not intended? (i.e. basically
   ignore them - they don't have sufficient precision). I presume so.

3. If ROFF isn't used by device support, can I just set it at
   initialization - or even in dct (this would require a change to
   aiRecord.ascii) to a specific value without breaking things?

I admit it is a bit ugly, but the alternative is to write a new analog
input record for the special case of a circular encoder - and still
have to write device support for it. If people think this is a better
way (or have any other suggestions), do tell me.

Nick Rees

Joint Astronomy Centre                    Ph:       +1 (808) 961-3756
660 N. Aohoku Place                       Fax:      +1 (808) 961-6516
Hilo, HI.  96720                          Internet: [email protected]

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System