[Gb-ccb] forwarded message from John Ford

[John Ford]

I agree that we don't need to timestamp every sample, only the
completed 1 ms integrations.

I think that this does need to be coordinated so that the data taking
starts at a precise time decided.  Here's my suggestion:

1) The manager issues a command to the backend to start on the next 1 PPS
tick (we're feeding that to the computer for timing anyway)

2) The digital hardware is armed and the state machine generating the
phase switching is started and data collection is started on the next 1 PPS
tick.  This would allow us to timestamp the data to the 1 ms precision.

Alternatively, the manager can issue a command to the backend to start taking
data immediately, and we live with whatever indeterminancy there is in
that transaction (a few 10's of milliseconds).  The USB transactions
are not going to be real-time, so the data really needs a timestamp,
or sequence number, at the source.

John


Tim Pearson writes:
 > I should try to clarify. In normal data-taking, our goal is to 
 > timestamp the data to at least millisecond precision, e.g., by counting 
 > samples from the last 1-second tick. My point about dump mode was that 
 > we don't *need* to timestamp every 100-ns sample to ns precision, 
 > although if it's easy we should do the best we can. If a dump is 
 > triggered by a command from the operator, then assigning a precise 
 > time-stamp might be difficult. The *minimal* requirement for the dump 
 > mode is a contiguous sequence of, say, 20K 100-ns samples, starting (or 
 > ending) within ~ 1s of the operator's command. If you see any need to 
 > synchronize the dump with some other event with high precision, then 
 > let me and Martin know.
 > 
 > - Tim
 > 
 > On Jan 22, 2004, at 5:02 AM, Richard Lacasse wrote:
 > 
 > > Tim,
 > >
 > > I wonder if at some point in time we may want to tie the data to the 
 > > instantaeous telescope position with higher accuracy than 1 second can 
 > > afford. The gbt is fairly big and floppy and we are outfitting it with 
 > > devices like the quadrant detector which can give an instantaneous 
 > > position for part of the telescope at a 10 Hz rate.
 > >
 > > Rich
 > 
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