Experimental Physics and
Industrial Control System

"Jeff Hill" <[email protected]> · Tue, 29 Jul 2003 08:34:09 -0600

Bill,

One possibility is a register synchronization glitch. If there are two registers
that contain the time, one which is fast changing, and one that increments once
each time the fast changing register rolls over, then if you read the fast
register just before the rollover and the slow register just after rollover then
you might see a discontinuity in time. The solution is usually to set a bit in
the control register before reading the time that prevents register updates
while you are reading them. Your manual for the board will probably have the
details.

Are you using a driver that plugs into drvTS.c so that the timestamps in the
EPICS records are based on the Datum bc635VME?

Jeff

> -----Original Message-----
> From: Bill Cruise [mailto:[email protected]]
> Sent: Monday, July 28, 2003 7:07 PM
> To: [email protected]
> Subject: Datum (Bancomm) GPS glitches
> 
> Hi,
> 
> We are using a Datum TymServ(tm) 2100 Network Time Server with GPS
> receiver option to provide our observatory with stratum one NTP.
> 
> In our IOC we have a Datum bc635VME Time and Frequency Processor.  This
> is basically the same board which some other observatories use, only
> most have GPS receiver options and ours gets the time from the 2100
> by means of an IRIG connection.
> 
> We use the receiver to directly read time from the internal registers
> via the VME buss, using direct calls.  This is done from inside a
> genSub record, and is the basic timing for our whole system.  We do not
> use any "real" drivers, and do not use the timing capabilities of the
> board.  We just read the time -- NOW.
> 
> The problem is that we have seen some timing glitches of 1 second and
> 10 seconds.  Last night there were four 1 second glitches.  The time
> seems to be 1 second fast on one read, and then OK 0.050 seconds later.
> This happens very rarely, and so far hasn't caused any real problems,
> but the capability seems to be there.
> 
> Has anyone else encountered any such glitches in reading time from a
> Datum GPS?  If so, have you developed a solution?
> 
> Thanks for any help,
> 
> Bill
> 
> --
> 
> William L. Cruise                             [email protected]
> Electronics System Supervisor                   FAX:    808 885-7288
> Canada-France-Hawaii Telescope                  Voice:  808 885-3121
> 65-1238 Mamalahoa Highway         http://www.cfht.hawaii.edu/~cruise
> Kamuela, HI   96743

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System