Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!texbell!merch!cpe!adaptex!neese
From: neese@adaptex.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: ESDI or SCSI or RLL?
Message-ID: <27800002@adaptex>
Date: 6 Apr 90 14:49:00 GMT
References: <18758@boulder.Colorado.EDU>
Lines: 50
Nf-ID: #R:boulder.Colorado.EDU:18758:adaptex:27800002:000:2775
Nf-From: adaptex.UUCP!neese    Apr  6 08:49:00 1990


> Well, most people out there seem to think that SCSI is the fastest and edsi
> gets you more bang for the buck.
> I saw an advertisment for Jameco: EDSI $169.95  @  15m/bits / second
>                                   SCSI $349.96  @  10m/bits / second
> seems like ESDI is better all the way around!
> 
> any comments?

First of all, you can't really judge performance of a SCSI drive by the data
rate off of the head.  With ESDI it is a good indication as the data rate off
the head is also the data rate to/from the controller in the CPU.  With SCSI
the bus is a bus that has defined timing requirements for handshaking that
allow data to run around on it at up to 5MBytes/sec (50m/bits / second).

  The operating environment and the types of devices to be used should dictate
which way to go.  A single drive system will generally be better off with ESDI.
But if it is a system that will also need a high capacity tape backup system,
SCSI may be the better choice as you save the cost of the tape card.

  If you are running a multi-tasking or multi-user OS, then SCSI is a far
better choice, if you use a decent adapter in the system.  (The ST01/02 does
not qualify).  SCSI has the ability to access more than one device at a time
thereby increasing throughput to a much higher level than ESDI is capable of.
This also assumes you have selected some decent SCSI devices as well.  As the
data rate is not a fixed rate on the bus, you can have data throughput anywhere
from 450KB/sec to 3MBytes/sec, at the BIOS level.  Using the 154xA adapter,
for instance, the bottleneck will always be the device on the bus, as the
adapter can run steady data rates of 5MBytes/sec in its slowest configuration.

  Unfortunately, you can't rely on the device manufacturer's data rates they
spec, as they are best case rates, with no bearing on reality.  Experience
has shown me that the only way to know what the real data rate is, is to hook
it up and test the living daylights out of it.  For the typical user, this
cannot be done obviously.  This is one reason I will make recommendations,
when asked, concerning the best SCSI devices around today.  There are also
things the typical user can't know.  Like, how well the device adheres to the
SCSI standard.  This is a real bag of worms.

  In selecting SCSI, software support should be a major consideration as well.

  Sounds like ESDI is a better choice from the nightmarish picture I painted
doesn't it?  Not really.  It isn't that bad.  I just think when making a choice
it is best to be armed with the facts.  And I make no attempt to color things.

			Roy Neese
			Adaptec Central Field Applications Engineer
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