Thanks for the kudos, Scott. Your observations about the applicability = are correct. As a net topology goes, it's an all-day sucker. Mike -----Original Message----- From: Newton, Scott [mailto:Scott.Newton@xxxxxxxxxxxxxxxx] Sent: Thursday, February 26, 2004 1:14 PM To: Brown, Mike (Austin, TX); weirsp@xxxxxxxxxx; gtang@xxxxxxxx; dbostan@xxxxxxxxx Cc: si list Subject: RE: [SI-LIST] Re: Middle (Star) Termination Mike, I agree with you.=20 The star connection is similar but not the same. The star connection has = the resistors at the middle of the star with transmission line = connections at all legs. This arrangement may be used where any leg can = be the driver. It's typically only useful in a synchronous system where = the value at the receiving legs can be sampled after any appreciable = reflections have decayed. One wouldn't want to use this as a clock = driver termination because of the slow edge, step-like profile of the = wave, etc. Thanks for the great ASCII schematic and synopsis! Scott -----Original Message----- From: Brown, Mike (Austin, TX) [mailto:mibrown@xxxxxx] Sent: Thursday, February 26, 2004 10:51 AM To: weirsp@xxxxxxxxxx; gtang@xxxxxxxx; dbostan@xxxxxxxxx Cc: si list Subject: [SI-LIST] Re: Middle (Star) Termination There are a couple of different concepts being mixed together here, I = =3D think. A driver driving multiple series terminated lines isn't quite the same = =3D thing as the star termination. A low-impedance driver can launch a half-amplitude wave into the load = =3D t-lines with proper selection of series termination Rt: =3D20 Rt Vs --/\/\/\----+-----/\/\/\------ Zo =3D20 Zs | . =3D20 + . =3D20 | . =3D20 +-----/\/\/\------ Zo =3D20 =3D20 The condition to launch Vs/2 is (Zs + (Rt/n)) =3D3D Zo/n, =3D20 with the limit on n such that Zo/n >=3D3D Zs. The superficially similar topology for the star termination is: Zo Rt Rt =3D20 Vs --/\/\/\---/\/\/\----/\/\/\----+-----/\/\/\------ Zo =3D20 Zs ^ | . =3D20 A + . =3D20 | . =3D20 +-----/\/\/\------ Zo = =3D20 =3D20 A match at node A requires (Rt + Rt/(n-1)) + Zo/(n-1) =3D3D Zo =3D20 As given earlier in the thread, Rt =3D3D Zo*(n-2)/n. =3D20 This leads to at most a half-amplitude signal at node A, which is =3D further attenuated by the star such that the wave launched into the =3D rightmost Zo's is considerably smaller than 1/2 amplitude: in fact, it = =3D is 1/4 for n=3D3D3. =3D20 Rt can be reduced to increase the launch amplitude, at the expense of a = =3D negative reflection coefficient at node A. There's proably an optimum = =3D reflection coefficient at that node which minimizes settling time to =3D some specified voltage at the far end of the load lines. I haven't read = =3D Grebenkemper's paper; I would not be surprised if he has analyzed it. The slow risetimes discussed in this thread are due more to the low =3D launch amplitude into the destination nodes than to any ZoC time =3D constant at the far end of the lines. Regards Mike -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir Sent: Thursday, February 26, 2004 1:31 AM To: gtang@xxxxxxxx; dbostan@xxxxxxxxx Cc: si list Subject: [SI-LIST] Re: Middle (Star) Termination George, 1) I have never found routing density to be an issue. But with better = =3D and=3D20 cheaper drivers, the star gets less use these days than it used to. 2) 5pf looking into 50 ohms of a series terminated source will always be = =3D slower than looking into 25 ohms parallel termination. I don't see =3D where=3D20 the number of loads that the source drives, assuming it is matched = is=3D20 visible at the far end of the transmission line. 3) Did the PTO approve a patent on a signal source with multiple loads? = =3D I=3D20 would like to see someone try and enforce such a patent. I think that = =3D is=3D20 silly as the Palm space multiplexer patent that is drawing howls of =3D laughter. Regards, Steve. At 10:17 PM 2/25/2004 -0800, George Tang wrote: >The down side to the star termination is shown below: > >1. The trace routing density increases drastically, since you need to = =3D route >a separate trace from the (electrical) center point to each load. >2. This works well for the ideal case when the loads behave as open >circuits. But if the load capacitance is 4 or 5pF and the driver puts = =3D out a >sub-nanosecond rise time, the wave form at the receiver will have a >significantly slower rise-time. >3. The star termination is patented, so you may need to pay a fee to = =3D use it. > >With some clever engineering, you might be able to come up with a =3D custom bus >design that eliminates the above problems. > > >-----Original Message----- >From: si-list-bounce@xxxxxxxxxxxxx >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Dan Bostan >Sent: Wednesday, February 25, 2004 1:50 PM >To: Scott.Newton@xxxxxxxxxxxxxxxx; Robert Haller >Cc: si list >Subject: [SI-LIST] Re: Middle (Star) Termination > > >I think this termination technique is well explained >in the Motorola data sheet for clock drivers driving >multiple loads. >/dan > >--- "Newton, Scott" <Scott.Newton@xxxxxxxxxxxxxxxx> >wrote: > > All, > > > > I haven't found the paper yet, but I've realized how > > the star =3D3D > > termination works, at least at a high level.=3D3D20 > > If anyone is interested, here is a summary:=3D3D20 > > Resistors with identical values are connected at the > > junction of the =3D3D > > star. The value is calculated such that the resistor > > of the driven leg =3D3D > > plus the parallel combination of the resistors in > > the undriven legs plus =3D3D > > the parallel combination of the impedances of the > > undriven leg traces =3D3D > > EQUAL the impedance of the driven leg trace. The > > resistor value is =3D3D > > calculated as (N-2/N)Zo where N is the number of > > legs in the star and Zo =3D3D > > is the characteristic trace impedance. The > > equivalent circuit yields a =3D3D > > matched impedance (almost) on the driven leg to the > > parallel =3D3D > > combinations of all of the undriven legs/resistors. > > A half wave is =3D3D > > propagated at the driven resistor (side closest to > > the driver). When the =3D3D > > wave hits the receivers, it doubles as it reflects > > (off of infinite =3D3D > > impedance juncture). The reflected wave propagates > > back to the driver =3D3D > > leg which looks almost like an infinite length > > matched impedance, so =3D3D > > very little reflection occurs. There are of course > > details such as the =3D3D > > output impedance of the driver, etc that are not > > covered in this =3D3D > > summary. > > > > Scott > > > > > > -----Original Message----- > > From: Robert Haller [mailto:rhaller@xxxxxxxxxx] > > Sent: Wednesday, February 25, 2004 12:54 PM > > To: Newton, Scott > > Cc: si list > > Subject: Re: [SI-LIST] Middle (Star) Termination > > > > Scott, > > John Grebenkemper wrote an excellent paper > > that is available in =3D3D > > the > > Designcon98 precedings "Network Topology Analysis > > Using The Reflection > > Coefficient" that does and excellent job of not only > > explaining the > > theory of middle termination but also presents the > > math. > > > > Regards > > Bob > > > > Newton, Scott wrote: > > > Can anyone please provide a brief explanation > > regarding the theory of =3D3D > > a middle termination scheme? > > > In the book" High Speed Digital Design" by > > Johnson&Graham the topic =3D3D > > seems to be glossed over. It essentially says that > > providing resistors =3D3D > > connected in the middle of a star network that have > > impedance of 1/3 Zo =3D3D > > work by halving the voltage at the node. I've used > > Hyperlynx to simulate =3D3D > > this arrangement and it works well. I've searched > > the web and haven't =3D3D > > found a satisfactory explanation. > > >=3D3D20 > > > Thanks in advance. > > > > > >------------------------------------------------------------------ > > > To unsubscribe from si-list: > > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' > > in the Subject field > > > > > > or to administer your membership from a web page, > > go to: > > > //www.freelists.org/webpage/si-list > > > > > > For help: > > > si-list-request@xxxxxxxxxxxxx with 'help' in the > > Subject field > > > > > > List technical documents are available at: > > > http://www.si-list.org > > > > > > List archives are viewable at: =3D3D20 > > > > > //www.freelists.org/archives/si-list > > > or at our remote archives: > > > > > http://groups.yahoo.com/group/si-list/messages > > > Old (prior to June 6, 2001) list archives are > > viewable at: > > > http://www.qsl.net/wb6tpu > > > =3D3D20 > > > > > > > > > -- > > Robert J. Haller (rhaller@xxxxxxxxxx) > > Principal Consultant > > Signal Integrity Software Inc. > > 6 Clock Tower Place, Suite 250 > > Maynard, MA 01754 > > Phone: (978) 461-0449, ext 15 > > > > >------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in > > the Subject field > > > > or to administer your membership from a web page, go > > to: > > //www.freelists.org/webpage/si-list > > > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the > > Subject field > > > > List technical documents are available at: > > http://www.si-list.org > > > > List archives are viewable at: > > //www.freelists.org/archives/si-list > > or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages > > Old (prior to June 6, 2001) list archives are > > viewable at: > > http://www.qsl.net/wb6tpu > > > > > > >__________________________________ >Do you Yahoo!? >Yahoo! Mail SpamGuard - Read only the mail you want. >http://antispam.yahoo.com/tools >------------------------------------------------------------------ >To unsubscribe from si-list: >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >or to administer your membership from a web page, go to: >//www.freelists.org/webpage/si-list > >For help: >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >List technical documents are available at: > http://www.si-list.org > >List archives are viewable at: > //www.freelists.org/archives/si-list >or at our remote archives: > http://groups.yahoo.com/group/si-list/messages >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > >------------------------------------------------------------------ >To unsubscribe from si-list: >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > >or to administer your membership from a web page, go to: >//www.freelists.org/webpage/si-list > >For help: >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > >List technical documents are available at: > http://www.si-list.org > >List archives are viewable at: > //www.freelists.org/archives/si-list >or at our remote archives: > http://groups.yahoo.com/group/si-list/messages >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.org List archives are viewable at: =3D20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =3D20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu