Fabrizio, I would agree with everything that Bob has suggested, and emphatically reinforce item 3. Every backplane connector model that I have had experience with has been modeled with partial mutual inductance for signal and power pins. If both sides of the connector are tied to the same ground node, (whether node 0 or some other node), then mutual inductances are shorted out, changing the characteristics of the model. For some connectors this change is huge and can amount to large differences in delay, impedance and coupling. Many board-level transmission line SI simulators utilize node 0 for simulation across all boards and connectors. This is, of course, incorrect and will lead to significantly incorrect results. There is nothing that can be done but to move to another nodal simulator like Spice. When you do this, and separate the grounds from each section of the design, then you do have to adhere to Bob's item #2. The reference for displaying your signals must be the local reference on the card, chip, or thing that you are measuring. Typically, all waveform displays will reference all signals to node 0. In order to utilize the remote reference you will either have to use some math with your waveform viewer and view the difference of the signal to the remote reference, or use a level shifting circuit element in the net list (my preference.). Finally, there are some connector models that are poorly constructed, having a large dynamic range between the largest and smallest inductive elements. This wrecks havoc in simulation of connectors with large mutual inductance matrices and will cause convergence issues or significant slow down in Spice performance (10 to 100:1) when compared with models from competitors. A particular connector I am thinking about was specifically designed for 2 to 3 Gig backplanes. When the grounds on either side of the connector are isolated (as they should be) then huge convergence problems occur, since this requires that a large number of extremely small partial mutual inductance elements be simulated. Tying both sides of the connector to the same ground will eliminate the issue, but with a severe loss in accuracy, and some still greatly increased simulation times. I hope this helps. regards, scott Robert Haller wrote: >Fabrizio, > I have a couple of points you should be aware of. >1. You do not HAVE to tie all of the ground nodes set to zero, but if >you don't >be prepared to accurately model your interconnect, connectors, decoupling >and signal to return ratio's. >2. When you are displaying an Eye pattern on a remote board not tied to >zero, >use the remote reference for your signals reference. >3. If you are using a connector model that contains partial inductances >you will violate >the assumptions of the field solver used to create that model if you >ground (tie to zero) >both sides of the connector. > >Regards, >Bob > > >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 > >Fabrizio Zanella wrote: > > > >>I have a question regarding backplane simulations of serial 8b/10b >>signals, running 2.5Gbs and above. Does it matter whether the ground >>nodes of the daughtercards and backplane are all set to 'zero' or to a >>floating node for eye pattern results? >>Thanks, >> >>Fabrizio Zanella >>Principal Hardware Design Engineer >>Broadbus Technologies >>fzanella@xxxxxxxxxxxx >> >> >> >> > > >------------------------------------------------------------------ >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 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 > > > > -- Scott McMorrow Teraspeed Consulting Group LLC 2926 SE Yamhill St. Portland, OR 97214 (503) 239-5536 http://www.teraspeed.com ------------------------------------------------------------------ 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 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