[rescue] Dead AXi?

[George Adkins]

On Friday 08 March 2002 12:03 pm, you wrote:
> I have an AXi machine here that seems to have trouble with cdroms.  I
> have the cdrom on a wide->narrow terminating converter.  If I try to
> boot cdrom, it takes about 5 minutes, and the cdrom looks idle...
> then it finally gives up on mounting the root fs.  Output follows:
>
A lot of people have trouble with wide-narrow conversions, let me offer a 
couple of pointers...

High-byte terminating convertors are best used for just two things:
1. (most useful) putting a wide drive on a narrow bus.  The termination of 
the high byte at the drive's interface does wonders on many drives to get 
them to negotiate down to narrow.
2. (less useful) reducing a Wide SCSI host adapter channel to narrow right as 
it comes off the card, assuming that the card is an older one that doesn't 
support high-byte autotermination well (or at all).

If you are going to share a bus between wide and narrow devices, the best bet 
is not to attempt to reduce the bus half-way down the run (in my experience).
Every adapter you add in-line creates impedence mismatches and generates 
signal reflections (however small they may be). the best bet is to cable the 
entire run as wide with active *external* termination at the end, and then 
put un-terminated wide->narrow adapters in between the ribbon cable and your 
narrow devices.  Yes, on-the-drive termination works, but I have had problems 
with it before, where I have never had problems with a separate active 
terminator.

This arrangement minimizes signal distortion at the far end of the bus, and 
provides the most sure and proper termination of the bus.

to Illustrate: (R=Ribbon, C=Connector)

C - SCSI Host adapter (termination on)
R
R
R
C - Wide SCSI Drive
R
R
C - Wide SCSI Drive
R
R
C - [un-terminated Wide-> Narrow adapter] - Narrow CDROM
R
R
C - [un-terminated Wide-> Narrow adapter] - Narrow Tape Drive
R
R
C - Active Wide SE Terminator

George
------------------
Suits  n. - those individuals who necessitated 
the addition of the word 'serial' to 'console'.