In a recent note, Roy Hewitt said:
> Date: Thu, 23 Mar 2006 12:42:46 +0000
>
> My understanding is that the timer just sends timing signals to the CPC,
> it is unaware of what the CPC is doing. Basically it is a one way
> process. Whether there is a CLO or not, the 9037 never gets the time
> from the CPCs. It just sends the time and stepping signals to control
> the CPC clock rate. (it also sends offsets for daylight saving/Leap
> seconds etc).
>
What initially sets the content of the TOD clock at IPL (or
is it POR) time. IIRC, there is some setting that can only be
accomplished at POR. Is there a clock in the HMC, so the
data path for that initial setting might be:
ETR to HMC (at POR)
HMC to TOD (at IPL)?
> Dave Kopischke wrote:
> >
> > And my understanding of the function of the timer is to steer the onboard
> > clocks on the processor. The processor clocks are always the source of time
> > information. The timer compares the time from the external source to the
> > time on the processor and either slows the clock down or speeds it up until
> > they match. If you don't have a CLO card, the timer itself is the time
> > source (your wristwatch is probably more accurate). You can't get more than
> > four seconds out of synch or the timer won't be able to generate the proper
> > steering impulses to get the clocks back in synch. There's lots of detail
> > in the manuals if you care to attempt to understand it.
> >
I recall we once got about 15 seconds out of sync, and were able
to recover without the dreaded POR by repeatedly issuing manual
commands to steer by a couple seconds until after about a week
the ETR was within tolerance to sync to NIST dialup. I believe
an IBM employee (Greg Dyck?) has supplied instructions for doing
this on this list.
-- gil
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