From: hyc@hanauma.jpl.nasa.gov (Howard Chu)
Subject: Re: How many SCSI devices/host adapter?
Organization: SAR Systems Development & Processing, JPL
Date: Mon, 5 Oct 1992 08:01:26 GMT

In article <16876C418.BWWL@ibm.rz.tu-clausthal.de> BWWL@ibm.rz.tu-clausthal.de (Wolfgang Ley) writes:
>In article <BvKEvx.Lxw@usenet.ucs.indiana.edu>
>dvasilef@silver.ucs.indiana.edu (David Vasileff) writes:
>>  This may be a trivial question, but I was wondering how many SCSI
>>devices I could hook up to my ST if I had an ICD AdSCSI host adapter.
>>For example, if I already had a 20meg HD with and AdSCSI, could I just
>>buy a syquest mechanism and plug it in (assuming I could fit it in the
>>case).
> 
>You can access the SCSI-IDs 0-7 with an ICD AdSCSI. If you have an adapter
>with a sepearte clock on it this will have the SCSI-ID 6. So 7 are left.
>If you want to use a second SCSI-Drive you just have to plug it in...
>(eh... due to the short cable inside my case, i had to install a newer (longer)
>one).
>...and don't forget to change the SCSI-ID of the second drive to another than
>zero (this could be done by jumpers on the HD).

The above looks correct. Just fyi, the "AdSCSI" has no clock, the "AdSCSI Plus"
does; this is the difference between them.

Normally with SCSI, you have 0-7 "target IDs" and each target can have 0-7
Logical Unit Numbers. So, you could theoretically have up to 64 devices on
your SCSI bus. In the bad old days (  }-)  ) you took an ST-506
hard drive and stuck an Adaptec SCSI controller onto it. The controller 
was given a particular target ID number, and it could control up to two
drives, so it would recognize LUNs 0 and 1. This was pretty common for
SCSI controllers, so now your 64 devices is limited to only 16 devices.
Nowadays most SCSI devices have "embedded controllers" which means the
controller comes with the drive and can only control that single drive.
This limits you to only 8 devices per SCSI bus, and the logical units 1-7
can't be used for anything. On other systems you're limited even further,
the computer's SCSi host adapter has it's own SCSI target ID, so only
7 other IDs are available for peripheral devices. No ST host adapter that
I know of reserves an ID for the adapter itself, although some do reserve
an ID for their onboard clock, as mentioned above. (Rather a silly thing
to waste a whole target ID on, I'd say. Oh well...)

Since it's not unreasonable for some folks to want more than just 7 or 8
peripherals on their SCSI bus, you can now get a sort of "bus extender"
from a few places. What these do is sit between a set of peripherals and
the main SCSI bus. The extender is assigned an ID, and any logical unit
numbers that go to it are handed out to the devices with the matching target
ID behind it. e.g.

		computer
		   |
		host adapter
		     |
		     | SCSI bus
		     |
		bus extender, ID 0
		  |     |            SCSI bus
		  |     +------------------------ ID y, LUN 0
     extended bus |
		  |
		  |
		--+--
          LUN 0 | | |  LUN x
      +---------+ | +------------ ID x, LUN 0
      |           |
      |           | LUN 1
   embedded       |
    drive        ID 1
    ID 0	LUN 0
    LUN 0

This setup brings your theoretical device limit back up to 56 (or 48)...
(The bus extender has to have its own ID on the extension bus.)
It seems these extender devices are only made for SCSI-2, perhaps they
felt SCSI-1 didn't have enough bandwidth to justify it.
-- 
  -- Howard Chu @ Jet Propulsion Laboratory, Pasadena, CA

California, land of dreams. For many, moving out to CA is their first step
toward fulfilling their dreams. For most of them, it's also their last.
Newsgroups: comp.sys.atari.st