## [SI-LIST] Re: Circle bus topology; Circular Firing Squad?

• From: olaney@xxxxxxxx
• To: steven.salkow@xxxxxxxx
• Date: Thu, 26 Jul 2007 15:29:21 -0700

```No, the waves in each direction have the same phase at the source.  They
would meet and *reinforce* each other at the electrical halfway point,
continue around the loop in both directions, and meet again at the
source.   Unless the source was back terminated, they would reflect from
the low source impedance, this time in inverted phase from the original
sense, and repeat the exercise until losses cause the waveform to die
out.  This is just a wierd example of exactly what happens on a regular
single ended trace with an open end, except that you are driving two
traces of identical length in parallel that have their ends shorted
together.  Also note that to back terminate the source, its impedance
would have to be half that of the transmission path, because it sees both
ends of the line in parallel.  Example: 50 ohm source to drive a 100 ohm
looped trace.  Or, you could terminate the halfway roundtrip point the
same way.  Or both, if you double the drive level (great for video but
not good practice for logic signals).  Basically, ring layouts in copper
foil are strange birds, useful for narrowband microwave purposes, but not
of much use for wideband, time domain waveforms for all the reasons
mentioned.

Orin Laney

On Thu, 26 Jul 2007 14:21:57 -0700 "Salkow, Steven"
<steven.salkow@xxxxxxxx> writes:
> Mr Townsend et All made good points.
>
> DFM software would flag this as an error.
> Electrically, I think the idea is that the propagating standing
> magnetic
> wave of each arc path would exactly cancel each other when the waves
> met
> and no reflections would reflect back to the source. If one side of
> a
> path was not exactly equal, the point at which the waves cancel
> would
> not be at an physical midpoint rather at an electrical midpoint
> which
> may fall on a driven node. The point made by others already is
> better
> schemes exist already such as star distribution where end point
> termination may be used to easily terminate a line with quite
> predictable results.
> ss
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Townsend, Fred
> Sent: Thursday, July 26, 2007 9:05 AM
> To: David.Carney@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Circle bus topology; Circular Firing Squad?
>
> David:
> I have to ask what would be gained from such a topology? Ron and
> Scott
> both make some very good points. In the case of the Mux bus
> (1553)the
> ring encompasses the airplane and the double ring structure gives
> increased reliability. Microwave can make use of some structures
> like
> directional couplers to help reduce reflections. If rings are good
> why
> aren't we still using the token ring? Again the token ring was over
> an
> area much bigger than a PCB. A ring in a PCB would have all of the
> problems with no apparent gain. Think about your PCB router. Rings
> would
> drive the router nuts?
>
> Fred Townsend
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of David Carney (Neenah)
> Sent: Thursday, July 26, 2007 6:47 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Circle bus topology
>
> Has anyone ever experimented with a circle bus topology.  The basic
> concept would be a bus with several devices attached.  They would be
> routed in a daisy chain topology, and then the two ends of the daisy
> chain would be connected together.  The PCB routing would look like
> a
> circle or a loop for each net on the bus.  Pointers to references
> such
> as papers or application notes would be greatly appreciated.  I'm
> particularly interested in signal integrity and EMC implications of
> this
> topology.
> =20
> Thanks.
> =20
> =20
> ----------
> David Carney
> Senior Hardware Engineer
> Plexus Corp.
> Phone - 920-751-5646
> =20
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