From: tweten@wilbur.nas.nasa.gov (Dave Tweten)
Subject: Re: EISA/MCA *or* Local Bus? Which is better for multitasking?
Organization: NAS Systems Division, NASA Ames
Date: Fri, 18 Dec 92 18:12:49 GMT

In article <1992Dec17.231538.29868@blkbox> rbarnett@blkbox (Rick Barnett) writes:
>goyal@utdallas.edu (MOHIT K GOYAL) writes:
>>I am curious as to which is better for OS/2, EISA/MCA or VL-Bus.(Vesa Local Bus)
>>Here is my setup:
>>single user on a stand alone pc.
>>I will be running lotsa programs..games, word processors, Pascal, Mathcad,
>>whatever else.
>>Anyways, I would like to know which would offer better performance.
>
>EISA is 32 bits wide with bus mastering but only runs at 8 MHz.  Video and
>hard disk controller cards are few and expensive.

My experience is that EISA disk controller cards are plentiful if
somewhat more expensive than equivalent ISA cards.  Mr.  Goyal says
he's most interested in SCSI disks.  The vendors of EISA SCSI
controllers with which I was most familiar when I bought mine about
nine months ago were Adaptec, Ultrastor and DTC.  The beauty of EISA is
if you don't want to pay the current premium on some card for EISA over
ISA (or even PC/XT bus) you don't have to.  Use an old card until the
prices come down to your comfort zone.

I should also point out that an 8.33 mHz EISA bus can transfer data at
33 megabytes per second in bus master mode (8.33 mHz x 4 bytes/cycle =
33.33 megabytes/second).

>VESA is 32 bits wide with bus mastering and runs at CPU speed (up to 50 MHz).

I don't think this is exactly true.  My understanding is that the VESA
specification requires VL boards to work up to 33 mHz, not 50.  It's
also important to note that the standard works only for three slots.

>>Also, does it even matter?  (ie-if EISA/MCA is much better, will it matter?
>>will I ever make full use of Local Bus, or vice versa?)

The function which will most benefit from a VL board is video
(remarkable, considering that VESA is a video standards group!).  EISA
gets most of its speed through parallel transfer and reduction in the
number of bus cycles per transfer after initial set-up.  Much video
work amounts to byte shuffling.  The VL bus's faster cycle time works
wonders when you just need to move one byte per transfer.  By contrast,
EISA shines when you need to move blocks of data to/from several I/O
boards (say, an ethernet, a single-ended SCSI, a differential SCSI and
an ESDI).

>>I plan on having a scsi-2 hard drive and a scsi cd rom.
>
>I have seen specs on two VL-Bus SCSI controllers, with data transfer rates
>up to 66 MBytes per second - since SCSI-2 maximum is 10 MB/sec, you can hang
>a few drives on it without saturating the I/O channel.

Any non-wide SCSI-II controller is limited by the SCSI spec. to 10
megabyte per second transfers over any reasonable amount of time.  A bus
mastering EISA SCSI controller will chop that 10 mB/s stream into 33
mB/s bursts.  A caching controller can deliver data already in the
cache at burst speed, which is probably where Mr. Barnett's number comes
from.

Mr.  Barnett is right when he says, "you can hang a few drives on it
without saturating the I/O channel," if he means the SCSI bus.  If you
get a true Fast SCSI-II disk, it will burst its data across the SCSI
bus at 10 mB/s, even though the data rate at the disk heads is more
like 3-5 mB/s.  That permits one SCSI bus to support two or three
solidly busy disks.  Be careful, though.  Most advertized SCSI disks
aren't Fast SCSI-II.  A slower disk can drag the SCSI bus down to 1.5
mB/s while it deals with that disk.

Neither Mr. Barnett nor I have mentioned MCA bus to this point.  I
wouldn't consider it.  IBM made a big mistake by initially charging
relatively large royalties for its use.  The result was EISA.  The
market has passed MCA by.  Low volume means few choices and a
continuing premium for MCA cards.

For what it's worth, my ideal motherboard would run at 33 mHz and have
EISA slots with one of them additionally capable of supporting VL
bus (for my video card, naturally).  If I had just won the lottery, the
CPU chip would be a 486DX2-66, otherwise a 486DX-33.  Such boards are
currently available.

>Hope this helps...
>Blair R. Barnett       u5c95@lfhp113.hso.link.com

And so do I.
-- 
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Dave Tweten						tweten@nas.nasa.gov
NASA Ames Research Center, M/S 258-5			     (415) 604-4416
Moffett Field, CA  94035-1000				FAX: (415) 604-4377
Newsgroups: comp.os.os2.misc