Adding a second SCSI disk

Adding a second SCSI disk

Post by Roger C. P » Tue, 03 Aug 1993 14:59:18

>I am about to add a second internal SCSI disk drive to my controller.
>The first one has SCSI ID 1 and the second one arrived set to SCSI ID 6.
>(I presume there is no problem with that.)  My question is:
>How do I tell DOS that I have a new disk drive, and how will a drive
>letter be selected for it?  I can't find any evidence of how DOS
>was told that my first SCSI drive exists and should be drive C:.
>The reason it doesn't seem possible for DOS to figure this out
>automatically is that, with SCSI disks, you tell the BIOS that you
>don't have any hard drive C: or D:.  So it is unclear how DOS finds
>out about such drives.
>Can anyone give me a clue here?

MS DOS versions prior to V5.00 could only handle a maximum of 2
physical hard drives without a device driver in CONFIG.SYS.  V5.00 and
V6.00 support up to 8 physical disks.  DOS determines the physical
disks via INT 13h which means the SCSI adapter must have a BIOS capable
of advertising greater than 2 hard drives.  Make sure your adapter can
do this.  The limit of 8 may be only FDISK's limit and DOS itself may
handle more.

Quote:>Secondly, the manual tells me that the first and last device on
>the SCSI cable must have termination resistors, and the rest must
>not.  Is this really true?  The same manual told me that this was
>true about two floppy drives on the same cable, and I ignored this
>and they both work fine.  But maybe SCSI devices are more sensitive
>to this rule?  Anyone have any experience with installing multiple
>SCSI devices?

Yes, it is really true for SCSI at least.  I've never paid any
attention to floppy drives though, but I am experiencing some wierd
problem right now which I will look into the next time I open up the

I had a SCSI setup which was working fine for a long time, then I moved
my monitor and put a fan on my desk (it was a hot summer day).  I
couldn't boot my system anymore.  I was baffled.  I made no system
changes to the SCSI bus, no hardware changes, no CONFIG.SYS or
AUTOEXEC.BAT changes.  I called in a co-worker, and he immediately
noted the problem.  The last hard drive on my SCSI bus was never

Moral: Just do it.  It may save you grief later.  I now have 2 SCSI
adapters, 2 hard drives, 1 magneto-optical, 1 DAT, 1 CD-ROM on the same
SCSI bus.  All correctly terminated and no SCSI related problems so far
(just driver problems :-).


PS: Do not connect 2 SCSI adapters to the same SCSI bus unless you know
exactly what you are doing.  DOS and all other OS's expect each drive
letter to be unique.  I juggle with device drivers to allow all hard
drives and optical to be controlled by one adapter and DAT and CD-ROM
to be controlled by the other.


Adding a second SCSI disk

Post by fred j mccall 575-35 » Thu, 05 Aug 1993 05:45:48

Quote:>Moral: Just do it.  It may save you grief later.  I now have 2 SCSI
>adapters, 2 hard drives, 1 magneto-optical, 1 DAT, 1 CD-ROM on the same
>SCSI bus.  All correctly terminated and no SCSI related problems so far
>(just driver problems :-).

And if it doesn't work, then UNdo it.  My SCSI chain fails if I
terminate it properly -- write errors, device problems, etc.
Unterminate one and and it works fine (by 'fine', I mean I'm getting
2+MB/s transfer rates with one drive and 750k/s on another (much
slower) drive, with everything else on the chain able to talk.

"Insisting on perfect safety is for people who don't have the balls to live
 in the real world."   -- Mary Shafer, NASA Ames Dryden


1. Adding second IDE hard disk

Just a few notes on IDE drives.

When there are 2, 1 must be master and the other slave.  You can't have one
be a slave and the other not specified like it is with only 1 drive.

Did you set up the CMOS tables so that your BIOS setup knows that there
are 2 drives?

I know that these are so basic, that you've already covered them, but without
a manual for the specific drives/card that's about all I can say.

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