\medskip

{\small

\memo{Revise and partially scrap this} 
Usually, at the heart of a phase-locked loop there is a
variable-frequency oscillator (VFO). The phase difference between the
oscillator and the reference signal is used to pull the oscillator's
frequency in such a direction as to reduce this difference. However,
since the phase is proportional to the integral of the frequency
rather than the frequency itself, one obtains in this way a {\em
second-order} loop; such a loop imposes certain trade-offs between
stability and agility of response.\footnote
	{Intuitively, in a second-order loop the phase has {\em
inertia} that needs to be damped. If is overdamped, it will never
reach the mark; if it is underdamped, it will overshhot it.}
	On the other hand, suppose that a signal having the desired
\emph{frequency} were distributed to all boxes, reaching each box with
a different phase (owing to propagation delays through the
distribution network). Then the secondary master could match its phase
to that of its neighbors by means of a \emph{first-order} loop where
the phase is adjusted directly (rather than indirectly through the
frequency), with improved response characteristics. Alternatively, the
distributed reference frequency could be used to adjust the VFO
natural frequency, so as to minimize the amount of error feedback
needed to pull the oscillator into phase lock.

Though the details of the local masters' strategy for achieving phase
lock are left unspecified, a single reference frequency may be made
available to all boxes through a \bus\ \tsy{Master Clock} signal
originating from the \cam\ controller.  This signal could be used
directly as the local master clock by the corner box, which would
ignore phase feedback from its neighbors and would thus force them to
do all the necessary adjusting.  By directly using a copy of the {\sym
Master Clock} as a local master, ``bare bones'' boxes designed to
work only in small machines (one to eight boxes) could dispense with
the phase-locked loop altogether.

\medskip

It may be decided at a later time to use the oscillator of the corner
box as the primary master. In this case, on the \bus\ trunk the {\em
Master Clock} signal would run from the corner box to the controller
(rather than the other way around).

}% end \small



----------------------------------------------------------------
Though implementation details for transmission-lines, line-drivers,
and line-receivers are left, for the moment, unspecified, we shall
assume that each transmission line consists of a \emph{pair} of wires
(such as the two elements of a twisted-pair line, or core and shield
in a coaxial cable).

----------------------------------------------------------------

If a reference clock signal is distributed along the
\bus\ (cf.\ end of \sect{interbox}), the corner box may send rather
than receive this signal on its \bus\ root port, thus clocking the
\cam\ controller rather than being clocked by it.
 

