From: markw@antimatr.hou.tx.us (Mark Whetzel)
Newsgroups: comp.sys.ibm.pc.rt
Subject: Re: SCSI-DISKS
Summary: Ah.. These more open systems have spoiled people
Date: 6 Jan 93 06:04:12 GMT
Organization: StarTrek and Lionel trains fan, Missouri City, TX

In article <C0E5wM.943@news.cso.uiuc.edu>, shair@barolo.cso.uiuc.edu (Bob Shair) writes:
> mark@physchem.ox.ac.uk (Mark) writes:
> >In article <1992Dec27.195328.125@vms.huji.ac.il>, allon@nesher.fh.huji.ac.il (Allon herman) writes:
> >> I am trying to install a non IBM scsi disk on my RT (running AOS 4.3).
> >> I am also aware that there has been recently a discussion regarding this
> >> topic in this newsgroup. I will be most thankful to the person who can
> >> tell me if and where can I find old articles that were posted on this
> >> newsgroup.
> >
> >The fact that it is a not an IBM SCSI disk shoulod have no effect.  SCSI is a
> >standard.  If it is a SCSI drive and it does not work with a SCSI
> >controller, one of the devices does not meet the SCSI standard.

Not true.. As Bob Shair said below, ANSI standards do allow for two electricly 
incompatible interface styles.  Also, different adapters have different
maximum data transfer rates, and not all adapters supported async and 
syncronous transfer modes.  Most modern controllers do now, but that
was not always the case.  The RT adapter supported both transfer modes
and up to 3.5M Bytes per second.  I believe that this was improved on
later versions of the IBM SCSI adapter.
 
> Let's not forget the difference between Differential SCSI, as implemented on
> the RT and several minicomputers, and Single-Ended SCSI, on the PC, Mac,
> RS/6000 and other workstations.  They are not electrically compatible;
> the only difference which I understand between them is that Differential
> SCSI allows longer distances (the RT SCSI disks were in racks external
> to the cabinet).
 
Single-ended SCSI uses only one signal wire for each control signal.
Differential-SCSI uses two wires in a push-pull driving pair for each
control signal.  This allows better noise immunity, and the longer driving
distance from the computer.. up to 25 Meters.

> I've heard that there are 3rd-party converters (black boxes?) which allow
> these to be intermixed, but have no experience with them.

True.  The boxes and internally mounted converter boards from
Rancho Technology work quite well for this task.  There are also at least
two other vendors making single<->differential converters that I know about.

I have used non-IBM disks on an RT running AIX 2.2.1.
I have run HP 9" disks, Maxtor 5.25" disks, and Fujitsu 5.25" SCSI disks on
an RT running AIX, as well as other SCSI disks from several manufacturers.

I also have running today at my office, HP 88780A 9-track tape drives,
Exabyte 8MM tape drives, and Fujitsu 3480 compatible tape drives on several
of our RT's running AIX 2.2.1. We had to write our own driver for the tape
drive support.

AIX and VRM can be tricked into running OEM disks, without too much trouble.
While it is awkard to get the OEM SCSI disks initially running, once you do,
they work fine, just stay away from drives bigger than 1 Gigabyte.  
VRM just gets completely lost and wraps accesses over 1 Gig back to
cylinder 0, destroying the minidisk partition table.  I believe this to
be a bug in the SCSI driver software, not a hardware problem.

People need to understand IBM's methods when the IBM RT was developed.
You buy a IBM computer, you get IBM devices to attach to it.  This was true
of several major makers of computer equipment at that time.

IBM only supported for attachment by the SCSI interface (at least for AIX,
not sure about AOS), the IBM 9332 external disk drive on the RT. 
The disks came in either 200 or 400 MByte capacity drives. We had an RT 125
on evaluation once, and it had one of the external table top 9332 drives on it.
They were monster drives with 11 inch in diameter platters, and were
available in standalone table top cabinets, or rack mounted. 
The internal drive electronics for those drives had special extended command
sets that supported IBM diagnostics, and also the ability for the RT to
download new microcode to the drive.  The drives came preformatted with
VRM partition table information already written to the drives.

The drives can be only used for DATA filesystems, /, /usr, /tmp and /u
filesystems were not allowed to be placed on the SCSI disks.
Initial IPL support did not understand the SCSI controllers.  The
"varyon" command brought in the proper support for the SCSI disks. into
the running AIX kernel.

Later,
markw
-- 
M. Whetzel 
Home: markw@antimatr.hou.tx.us        | IBM RT/125 running AIX 2.2.1 
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