Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!nosc!ucsd!rutgers!gatech!sbmsg1!scbhq!uahcs1!madhat!alvitar
From: alvitar@madhat.UUCP (Phil Harbison)
Newsgroups: comp.sys.ibm.pc
Subject: Re: NuBus
Message-ID: <273@madhat.UUCP>
Date: 5 Jul 88 20:49:59 GMT
References: <26.22CD76D1@ankh.UUCP>
Organization: DataVision, Huntsville AL
Lines: 31

In article <26.22CD76D1@ankh.UUCP>, Frank_Hill@ankh.UUCP (Frank Hill) writes:
> I heartily agree that the MCA buss should *NOT* be endorsed. ... The
> TI NuBus ... would provide an ideal buss architecture ...

I  fully agree with Frank.  I've designed cards for several busses (VME,
PC/AT,  Q-Bus,  MultiBus)  and  thoroughly researched most of the others
(except  FutureBus).  NuBus is clean, simple, and processor independent.
It  supports  automatic  configuration  with  features  like  geographic
addressing and identification PROMs.  NuBus uses distributed arbitration
and  directed interrupts, so there are no daisy-chain signals.  The only
centralized  services  are the reset and clock logic, and there are ways
to  distribute  even  those resources.  NuBus uses a multiplexed, 32-bit
address  and  data bus, with maximum bandwidth of 37.5 MegaBytes/second.
The data bus is unjustified, so there is no need for byte swapping logic
except  on  the  CPU  card,  where  it  belongs.   Nubus also provides a
"try-again-later"  response  to  a memory cycle, making it easy to build
adapters to other busses, or implement cache coherency schemes.

The  new  form  factor  used  in  the MacII (4.0" by 12.8"), is slightly
smaller  than  the  size  of the AT-bus cards (4.8" by 13.12").  Systems
built  around  this  form factor could probably use the rash of existing
cheap  PC  enclosures.   If  the  386  PCs  use  NuBus, then third party
hardware vendors can build a single product for both markets.

NuBus uses the 96-pin DIN-41612 connectors, which are more reliable than
the  edge  connectors  used  on  the AT and the MCA.  Pin density is 1.5
times  that of the connector used in the AT, and about the same as those
used  in  the MCA.  The connector is located closer to the center of the
board  (about  3  inches  from  the  end), which should simplify printed
circuit  board  layout.   Routing is complicated when all the connectors
are on one end of the board, as in the AT. 

By now you've probably figured out which bus is my favorite. :-)