Re: [SI-LIST] : Future bus backplane

L D Miller (L_D_Miller@BayNetworks.COM)
Fri, 05 Dec 1997 11:28:00 -0800

>From a noise standpoint, option (1) is greatly preferable, in that you have
at least two signal layers nominally referenced to a quiet ground (the top
two signal layers). Option(2) puts all signal layers opposite usually
noisier 3v and 5v planes. The degree to which this would affect you would
be dependent upon the actual layer thicknesses.

We find from our vendors that the difference in cost between 6 layers and 8
layers is pretty insignificant. If you use 8 layers, things get much better:

signal
ground
signal
thick core
power ground
thin core
power
thick core
signal
ground
signal

This gives you a buried capacitance layer (good to very high frequencies)
for the power, and at least 3 of the routing layers are referenced
(impedance-wise and return current mirror-wise) to primo clean ground
planes. The fourth routing layer (third from bottom) is at least some
greater distance from the noisy power plane.

Larry Miller
Bay Networks, Inc

At 09:17 PM 12/5/97 +1100, you wrote:
>We are designing a system which is derived from futurebus using the 2mm amp
>connectors. It is a synchronous time slice bus running at 12.5MHz. We are
>planning to distribute the clock from the centre of the backplane with each
>board having its own clock. Signal pins are alternated with ground pins
>across the connector. We are also using Futurebus transceivers for the
>address and data because of the control they provide.
>
>We are now considering how the backplane is to be laid out. One question is
>layer stackup. We need at least 6 layers. At a first pass we are
>considering either
>
>1.
>Signal
>Ground
>Signal
>very thick core
>signal
>Power
>signal
>
>or
>
>2.
>ground plane
>signal
>3V plane
>signal
>thick core
>signal
>5V plane
>signal
>ground plane
>
>Would you care to comment on these please given that we also need to have
>fairly uniform trace impedances
>
>I do not want alternating signal/plane layers as we will have some
>difficulties getting the backplanes manufactured.
>Garry Allen
>Australian Broadcasting Corporation
>Ultimo
><garrya@abc.gov.au>
>
>