Re: 2940

[Bob Doolittle <Robert Doolittle Eng Sun COM>]

I hope you don't mind me forwarding this to the list, Drew - it's an easy
and very dangerous mistake that I hope others can avoid.

> Date: Tue, 20 Oct 1998 10:09:46 -0700
> From: Drew Tennenbaum <drewpt earthlink net>
> To: Bob Doolittle <Robert Doolittle Eng>
> Subject: Re: 2940
> 
> With my setup I have
> 
> 2940UW
> UW SCSI Harddrive (Internal)
> SCSI Narrow CD-Rom (Internal)
> SCSI Narrow CDR (External)
> 
> Unless I turned off wide-negotiation in the bios, the SCSI module in Linux
> keeps resetting the line, and eventually doesn't work.
> 
> -Drew

This is an illegal configuration!  Go directly to jail, do not pass go!

This is very poorly documented in Adaptec's "quick install guide", but
go to the www.adaptec.com site FAQs and poke around and you'll see the
issue.  You've used all 3 connectors on the card.  You are only allowed
to use 2.  The "SCSI bus" is just that - a bus.  You've created a "Y"
topology which is no longer a bus, and are asking for Big Trouble, in
WIN/DOS or Linux.

I also have internal wide, internal narrow, and external narrow
devices.  However, I managed to catch the issue before wiring stuff up,
so I only use my 2940UW for the internal wide and narrow devices.  I
use a seperate, old 1542B controller I had expected to replace with my
2940UW for my external, narrow devices and this configuration works
great (except with cdwrite.  I use cdrecord instead)!  My only
question is whether I *had* to disable the BIOS on my 1542B.  My old
1542B docs said if you use two 1542's you have to disable BIOS on one
of them, but Adaptec tech support said I could have BIOS on both my
1542B and 2940UW enabled.  I had already disabled the 1542B and it's
more trouble to change it back so I just left it as-is, so I don't
know the right answer (except that my config works).

So, I'm afraid you have to take one of the below options (assuming you
want to use all your devices :-).
+ Buy another controller.  I'd get something relatively cheap for my
  external narrow devices since speed isn't an issue.
+ Buy a 64-pin to 50-pin internal SCSI cable adapter so that you can
  plug your internal SCSI 50-pin narrow CD-Rom onto the 68-pin wide
  internal SCSI bus, thus freeing up your internal narrow bus (and
  disconnect it from the 2940UW).  Make sure you buy an "internal"
  68-50 pin adapter, not an "external" one (which has termination for
  the high byte).  Presumably you're already using an external one for
  your external narrow devices since the 2940UW has only an external
  wide connector.  The bad news is that these cable adapters aren't as
  cheap as they should be.  Adaptec wants $30 for them, and my local
  Fry's electronics isn't much less (when they even have them in
  stock).
+ Replace your 2940UW with the new 2940U2W.  This allows you to use all
  three connectors (I don't exactly know why), and supports Ultra2 speed.
  There are several disadvantages to this choice, though - Linux
  doesn't support it yet, and it's wicked expensive (about 2X the cost
  of a 2940UW).
  
I have several internal narrow devices (CD-ROM, tape, and an old narrow
disk), so the cost for cable adapters was more of an issue for me.
I opted to keep my old SCSI controller in my system to use with my
external devices, and I'm glad I did.

Good luck, and let's be careful out there!

-Bob