DATMAN TECHNICAL BULLETIN #017
From: tech@datman.com
To: DATMAN user
Subject: SCSI Host Adapters for DATMAN
Date: 1996-06-05
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DATMAN claims that "any" ASPI-compliant SCSI host adapter can be
used. Unfortunately, not all SCSI adapters and ASPI managers are
created equal. This article discusses DATMAN's SCSI adapter
compatibility issues.
There are two components in SCSI interface as far as DATMAN is
concerned. One is the adapter hardware itself. We believe the
electrical characteristics of the SCSI interface is very straight-
forward that most major brands are designed well. The second
component is the ASPI manager which makes the lowest level
interface to the SCSI circuit. ASPI stands for Advanced SCSI
Programming Interface. ASPI was originally developed by Adaptec,
the front runner in the SCSI technology. Its definition was made
available to the public without charge by Adaptec. We believe it
is still tightly controlled by Adaptec. It is a de facto standard
of SCSI interface for PCs.
The good thing about ASPI is that most SCSI device drivers such
as DATMAN need not deal with countless variations of SCSI adapters.
Today, virtually all PC-compatible SCSI adapters are bundled with
corresponding ASPI Manager software. (Please note that we do not
supply ASPI Mangers. They come with SCSI adapters.) The DATMAN
File Engine sends SCSI commands (stored in memory) to the ASPI
Manager. Whether a particular SCSI hardware acts as a busmaster,
or it uses a certain interrupt (IRQ N) is not DATMAN's concern.
The bad news is that we found the weakest point in this architecture
is the ASPI manager layer. We have not discovered any serious
bugs in Adaptec's ASPI managers. After all, it is their creation.
On the other hand, we have seen ASPI Managers from other manufacturers
(such as NCR) having problems under certain conditions. If we were
to use only the simplest commands, we will probably not encounter
problems. However, in order to squeeze every bit of performance
out of the DAT drive which has a very long seek period, DATMAN
does use some advanced features built in the ASPI protocol, namely
the "ASPI Command Posting" method. The most notable case is DATMAN's
background volume mount operation that is carried out automatically.
This aspect of the ASPI protocol is not always implemented right by
SCSI adapter vendors.
For example, we found certain ASPI managers for NCR's SCSI cards
having problems with DATMAN. We suspect that posting an ASPI
command sometimes freezes the system.
NCR SDMS V3.0 SCSI CAM Driver DOSCAM-3.03.00 (works)
Symbios Logic SDMS V3.0 SCSI CAM Driver DOSCAM-3.05.00 (problem)
Version 3.03.00 written by NCR works with DATMAN and the newer
Version 3.05.00 written by Symbios Logic crashes he system.
If you have a SCSI host adapter that is not made by Adaptec,
please let us know about your experience (of both successful and
unsuccessful stories).