NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/comp/sys/ibm/pc/hardware/32844

Text File | 1992-12-11 | 3.4 KB | 68 lines

Newsgroups: comp.sys.ibm.pc.hardware
Path: sparky!uunet!spool.mu.edu!caen!zaphod.mps.ohio-state.edu!cs.utexas.edu!csc.ti.com!tilde.csc.ti.com!mksol!mccall
From: [e-mail] (fred j mccall 575-3539)
Subject: Re: Bus def's ==> Re: EISA vs. ISA (& VESA, VL-bus, Local bus)
Message-ID: <[e-mail]>
Organization: Texas Instruments Inc
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Fri, 11 Dec 1992 20:34:08 GMT
Lines: 57

In <[e-mail]> [e-mail] (W.R. Volz) writes:

>|> While we are at the subject, could some one please tell us all,
>|> what are the differences between
>|>     * ISA         (the mormal 386 type bus, right?)
>|>     * EISA        (extended ISA, 32 bit, look like IAS but twice
>|>                        longer right?)
>|>     * VL-bus      (differnt type of expansion slot, looks all
>|>                        diferent, much smaller than ISA, connected to the
>|>                        CPU directly, good for video cards, or is it VESA?)
>|>     * Local Bus   (?)
>|>     * VESA        (?^10)

>ISA is the old bus for the AT. It is 16 bits wide and runs at 8 mhz though
>it can be sped up some.

>EISA is a new Extended ISA bus. It is 32 bits wide and runs at 33 mhz,
>I think. This bus should be 32 * 33 / ( 16 * 8) or about 8 times faster
>than the ISA bus. I don't know what cards are available but ISA is
>compatible with this bus though is halves the bus speed. Don't know what
>that does to cards expecting an 8 Mhz bus.

People keep asking this question, and people keep getting it wrong.
EISA runs at ***8.33*** MHz.  It will transfer at FOUR times the speed
of the ISA bus (twice as wide and only requires one clock per transfer
vice two for the ISA), for a speed rating of 33 MB/s.  Plugging an ISA
card into an EISA bus does NOTHING to the bus speed.  Of course, that
card can only talk 16-bits wide, so THAT CARD will run at half the
speed of an EISA card plugged into the same bus.  Cards expecting an 8
MHz bus see an 8 MHz bus, because that's what the bus runs at (8.33
MHz).

>VL-bus is Vesa Local bus.

>Local Bus is a special set of up to 3 slots that connect directly to
>the CPU bus. It coordinates with the CPU for data transfer, this giving
>very fast response. This is a new design so things could change in the
>next year or so. Usually one puts a Local bus video card and a disk
>controller in the slots so you get very fast graphics and disk access.

>Vesa is a local bus standard that most vendors are appearing to
>adhere to now. It runs at 33 mhz. What that does for 50 mhz systems, I'm
>not sure. Someone say that one adds some stop bits on to the bus to
>slow it down????

Close.  It runs at UP TO 33 MHz (I've been told the actual spec goes
up to 40 MHz, but there are no 40 Mhz VL systems out there yet because
there aren't any 40 MHz CPUs except for the AMD 386.  The VL bus will,
obviously run at the speed of the CPU.  If the CPU is a 25 MHz part,
the VL bus slots will run at 25 MHz (not 33 MHz).  I'm not sure what
happens if the CPU is a 50 MHz part, either.

--
"Insisting on perfect safety is for people who don't have the balls to live
 in the real world."   -- Mary Shafer, NASA Ames Dryden
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[e-mail] - I don't speak for others and they don't speak for me.