🚀 go-pugleaf

>/* ---------- "ESDI 15/20 Mbit Drives on 10 Mbit C" ---------- */
>What happens when a 15 Megabits/sec ESDI drive is connected to a
>10 Megabits/sec controller?
>
>For example, what happens if I put a 760 Meg drive on a PS/2 with the
>IBM ESDI Fixed Disk Drive Adapater/A in it?  I know a lot about such
>installations, because I have have installed a Micropolis 1355, but
>that was a 10 Mbits/sec drive.
>
>If it works at all, will there be bad effects?  For example, from
>reading the Technical Reference I don't see any way to set the
>interleave on the IBM ESDI/A adapter.  Will very poor performance
>result because the interleave is too agressive?  Or will it just write
>to the disk at 10Mbits/sec, reducing the total space on the disk to
>10/15 of rated?
>
>I assume the same question applies to 20 Mbit/sec drives on the 15
>Mbit/sec controllers.

You cannot use a 15MBit drive on a 10 MBit controller.  The drive is the
one who dictates the data transfer rate to/from the controller.  The same
aplies to a 20MBit drive.  The worst thing hat usually happens is the
drive may appear to function for a while (2 or 3 minutes) and then data starts
to disappear.  The controller must be matched to the drive.  This is something
that anyone should consider before buying an ESDI controller/drive combination.
To be able to grow the capacity from 300MBytes to 760MBytes and beyond, you will
also have to buy another controller to go along with it.  This is one of the
many reasons so many people are switching to SCSI.  With SCSI the clock rate
of the drive makes no difference.  For instance, the latest batch of 1.2GByte
drives clock at 24Mhz.  SCSI doesn't care, it just trucks right along.  I
have had many compliants about this limitation of ESDI, but there is nothing
that can be done about it.  We have sort of tried to help things out with
the new ACB-2322D controller.  It will operate 10,15, and 20MBit ESDI drives
all mixed up on the bus, but when the 24Mhz ESDI drives become available this
won't do any good.  So you ask why not build the 2322D to handle it.  The only
way this can be done reliably is to have drive to test it with, so we are
stuck.  

			Roy Neese
			Adaptec Central Field Applications Engineer
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