Re: Understanding the time support on Epics.

[jbk@aps.anl.gov (Jim B. Kowalkowski)]

> 
> I am trying to understand the Time Stamp support under Epics R3.12.
> Marty Kraimers guide refers to the manual `Synchronous time stamp
> support' by Jim Kowalkowski, which I can't find on the Web.

Look at http://www.aps.anl.gov/asd/controls/docs.general.html.  Hit
on Synchronized Time Stamp Support (Postscript[71k]).

> 
> The problem I have is that I have a GPS clock on a VME crate and
> want to serve time from there to other VME crates, Vaxes and Suns on
> the network. It is particularly important that one Vax, in particular,
> has a pretty good knowledge of the time.
> 
> I was thinking of running the VxWorks port of Xntpd on the VME crate
> and then using that as a time server for all the network. The questions
> I have are:

> 
> 1. Can I do this?

The last I looked at Xntpd, it was huge and not supported for vxWorks 5.1.
It appeared quite beastly.  This was a while ago.  The soft time system
currently syncs time stamps using NTP to a unix server.

> 
> 2. How do I integrate this into the Epics/vxWorks time support?
> 

There is a fairly straight forward method to add your own timing software
on the bottom of the library.  It is described in the document.

> 3. Has anyone done anything similar?
> 

Yes, I think Andrew Johnson has (I could be wrong).  The APS uses the APS
event timing system for synchronous time stamps or the default NTP timing for
soft timing between IOCs.

> 4. What are the problems involved?

This depends on want you are attempting to achieve with timestamps.
Timestamps are filled in when a record processes.  If your timestamps
increments at a high rate and you are using the GPS to record the current
time in the timestamp, correlating events (record processing times) using
tools such as the EPICS archiver may be difficult.

This may not be a problem for you and I realize that I have not explained it
very well.  Send me email on how you plan on using the GPS and I will explain
some of the problem that I know about.

Jim

---------------
Jim Kowalkowski
 Argonne National Laboratory
 Advanced Photon Source
 708/252-9410
 jim@anl.gov
 jbk@phebos.aps.anl.gov