From: eje@irenaeus.mlo.dec.com (Eric James Ewanco)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: What is local bus?
Date: 3 Sep 92 16:27:46 GMT
Reply-To: ewanco@kalvin.enet.dec.com
Organization: Digital Equipment Corporation


In article <1992Sep2.163017.25644@cci632.cci.com>, sjo@cci632.cci.com (Steve Owens) writes:
> From: sjo@cci632.cci.com (Steve Owens)
> Subject: Re: What is local bus?
> Date: Wed, 2 Sep 92 12:30:17 GMT+5:00
> Organization: [Computer Consoles, Inc., Rochester, NY

> In article <1812u0INN51t@usenet.INS.CWRU.Edu> bk158@cleveland.Freenet.Edu (Su-Cheong Mac) writes:
> >
> >What is Local Bus? It seems to be a technique improving I/O performance.
> >Is there any article or on-line document which gives a clear description
> >on Local Bus? Thanks in advance.
> >
> >Mac Su-Cheong (MSC)
> >msc@sun4.ee.ncku.edu.tw

> 	Local Bus is another 32 bit bus architecture for clones that runs
> at processor speed.  That's about all I know, but I have some questions
> about a related topic:

There are two kinds of local bus: "local bus" and local bus ;-) What local bus
really means is that devices are connected to the processor's data and address
bus, just like the memory; the bus that runs at the full speed of the
processor.  This kind of bus does not have the contention management of an
expansion bus.  Or this is what I've heard, there may be some experts here who
will refute me.

The other kind of "local bus" is really simply a real expansion bus removed
from the local bus but which has contention management and other features of a
real expansion bus but which is 32 bits and runs at the full speed of the
processor (or a really high speed).  This kind of bus in reality is simply a
specialized expansion bus.  Because of the speed, the number of slots you can
put on such a bus is limited to about three or so.  An example of this kind of
bus is the VESA bus spec.  It is really a bus architecture like ISA or EISA but
designed to coexist with ISA and EISA.

> 	I read in Computer Shopper recently that VESA has developed a
> Local Bus standard that the industry is adopting.  Since I'm going to
> be in the market for a new motherboard within the next few months, I
> was wondering:

> 	1) What's the difference between Local Bus and EISA?

EISA is backwards compatible with ISA, runs at a fixed clock rate, has entirely
different specifications, and supports lots of slots.  VESA is a whole heck of
a lot faster, not bound by ISA backwards compatibility, and probably ties the
devices much closer to the processor than EISA.  VESA will support the true
throughput of the processor, that's my understanding.  I also believe VESA is
substantially simpler than EISA.

> 	2) I know the industry has only been luke-warm about adopting
> 	   EISA, but how do you think Local Bus will contend?

"Local bus" solutions such as VESA are important because they suffer none of
the disadvantages of EISA and are substantially faster.  VESA is also a lot
cheaper than EISA and will add very little to the cost of the computer (or the
expansion card).  The disadvantage is you can only have a maximum of three
slots else your computer must have a disc jockey license from the FCC! ;-) VESA
is designed to coexist with ISA and EISA.  However, what VESA _will_ do is
substantially diminish the reason for buying an EISA machine, because you can
now do things with VESA that previously required EISA, but didn't require eight
EISA slots.

For example, the main applications where ISA is inadaquate, and which EISA has
primarily been used for, are: memory expansion; network communication; graphics
cards; and disk I/O.  Usually the manufacturer will provide enough memory
expansion on the motherboard to obviate the need for a memory expansion card.
(Actually I am not sure an EISA memory expansion card makes any sense.)  That
leaves three applications, and, lo and behold, you can have up to three VESA
slots.  Any other application that requires the throughput of VESA/EISA is
likely to be very specific.  So if you only need high bandwidth for those
applications, then VESA and ISA will do you just fine.  Even if you do need
another application, more than likely one of those other applications you won't
need.  For example, a network server doesn't need fancy graphics, but does need
hard disk I/O and Ethernet.  Most people don't need high-speed Ethernet, so
that leaves one slot open for another application.

> 	3) Should I wait and purchase a Local Bus motherboard when they
> 	   become more abundant?

I think you should.  Do not go with anything but VESA or PCI (Intel's recent
standard), and I would prefer VESA.  VESA motherboards are about to flood the
market. 

> 	4) What about ISA?  Is it basically a dying breed, or is there
> 	   enough life left in it for me to consider purchasing a
> 	   motherboard with an ISA bus?

ISA is technically out of date and wholly inadaquate for today's uses.
Unfortunately so are a lot of things we can't do without!!!  ISA is far from
dead, for a long time coming you will see the vast majority of expansion cards
for ISA.  ISA is probably stronger than DOS!  But like DOS, it is out of date
but so prevalent you won't get away from it for a long time.  And with VESA
options, I see little reason, in Intel computers, to go to another bus.

However ISA poses a serious problem for Alpha PCs, because the Alpha processor
cannot do 8-bit or 16-bit data accesses :-( and ISA will _only_ do 8-bit and
16-bit accesses.  Alpha desparately needs a real bus, and although it will be
released with EISA, EISA will not do it justice.  Unfortunately the number of
PC cards designed for Turbo Channel is quite limited ;-)

> 	The processors I'm considering at this point are either a
> 486-25 or a 486-33, with a minimum of 8Mb RAM to start.  I'm not
> sure which manufacturer I'll look for, but I'm sure it will be a
> name I know (such as Mylex, AMI, or Chips & Technologies.)  The
> only thing I'm not sure of is bus architecture.

Zeos has just announced a very nice VESA machine, with a motherboard SCSI
controller on the VESA bus, and a 24-bit color VESA bus video adaptor (leaving
one more VESA slot).

> 	Anyway, any information is greatly appreciated.


> 					Steve
Eric
--
/=============================================================================\
| Eric Ewanco - Software Engineer     For the rash and outrageous opinions ex-|
| Digital Equipment Corp, Maynard MA  pressed herein I alone am responsible;  |
| ewanco@kalvin.enet.dec.com          they do not belong to DIGITAL(TM).      |
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