Subject: Re: Dual SCSI, single chain?
To: NetBSD/alpha Discussion List <>
From: Greg A. Woods <>
List: port-alpha
Date: 03/29/1999 02:13:30
[ On Sunday, March 28, 1999 at 19:31:23 (-0500), Curt Sampson wrote: ]
> Subject: Re: Dual SCSI, single chain?
> No, this is certainly not the case. You can mix any combination of
> stuff you want (excepting differential)--narrow, wide, slow, fast,
> ultra, etc., so long as you don't mind the bus contention (i.e.,
> a 4 MHz CD-ROM drive transferring 1 MB/sec is going to use a quarter
> of the bus bandwidth regardless of the width and speed of the
> controller and other devices on the bus).

It's not just a matter of bus contention, and you *cannot* just mix
things arbitrarily.  There are issues w.r.t. whether or not you've got
the devices in the correct order on the bus, whether or not you're
meeting the minimum cabling and connector standards for the highest
negotiated speed device on the bus, whether or not the upper and lower
halves of the bus are properly terminated, whether or not the attached
devices properly follow the specifications for option negotiation, and
finally whether or not you can properly program the host adapter to
control the various options on a per-device level.  Even once you got
everything right in theory there are some fairly tricky timing issues,
not to mention that SCSI-1 and SCSI-2 devices which barely meet their
own requirements are unlikely to meet the more stringent requirements
necessary for Ultra and Ultra-2 speeds (even if they're off at the
"narrow" end of a bus).

As Matthew Jacob's much more detailed answer suggests, it's much safer
to keep your slow and "fast" devices on separate buses.

> In fact, LVD is the only
> one you can't mix without losing capabilities (LVD devices revert
> to single-ended if there are any non-LVD devices on the bus), and
> differential is the only one you can't mix at all.

LVD (or any "differential signaling") is not equivalent to transfer
speed.  You're mixing your metaphors!  ;-)

Perhaps what's confusing here is that if you use Ultra2 then you "must"
use LVD signalling (or rather using single-ended signalling "is not

> Again, no. Virtually all but very old narrow disk drives are fast
> SCSI (10 MHz). A lot of CD-ROM drives appear to be slower, though
> (4 MHz).

I've got lots of "slow" SCSI drives (and man are they ever SLOW!).
They're sure a heck of a lot newer than my SMD drives, but yes, they are
quite "old".

FYI, "slow" is typically referred to as 5MHz, though in terms of the
standard this is relatively meaningless since the standard doesn't
mention things like signal "frequency".

I'd highly recommend anyone interested in these issues take a look at
Gary Field's "SCSI Info Central" (which includes the SCSI FAQ):

							Greg A. Woods

+1 416 218-0098      VE3TCP      <>      <robohack!woods>
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