From: Joseph R. Grace <JGrace@TetraSoft.com>
Subject: Re: HP DAT wont work with my system, SCSI problem?
Organization: TetraSoft International
Date: Sun, 22 Nov 1992 23:25:30 GMT

In article <By2ts3.Myr@newcastle.ac.uk> S.A.McIntyre@durham.ac.uk writes:
>[...]
>  sd1 (2,0) scsi_timer: timeout op: 0x28 st_state: 7 scsi_status 0x0
>  Target 3: BUSY; retry 1 ... 9
>  
>  
>  I have the following SCSI devices:
>  1 - Internal HD
>  2 - External HD
>  3 - CD Rom
>  4 - HP DAT
>  5 - HSD Scanner
>[...]

Scott,

I have what sounds like a practically identical set-up.  I have spent dozens  
upon dozens of hours eliminating problems with the SCSI devices (including  
swapping HSD Scanners and internal HDs).

My advice:

First, never buy a SCSI device which does not have an outgoing SCSI connection.   
It makes it (even) more difficult to diagnose and isolate SCSI problems.  (Too  
late for you and me, unfortunately.)  It's a major pain in the you know where  
and (I believe) a sign of poor quality SCSI device.

Second, I suspect your SCSI chain is too long!  There is a (little known?)  
limit of about 6m to the SCSI cables.  When you exceed this limit (or even  
start getting near it), devices start (intermittently) flaking out.   
Unfortunately, they don't just totally stop working and some are more  
accomodating of SCSI length than others.  My approach now is just to assume the  
SCSI cable/connector/length is broken or loose.  The devices, themselves, are  
"fine".  Unfortunately, the SCSI device vendors (at least the ones I've dealt  
with) are totally inept at debugging SCSI problems and are more willing to swap  
devices than diagnose and solve the real (SCSI) problem.  Perhaps there is *no*  
real SCSI diagnostic :-(.  (Market opportunity! :-)

Third, when buying SCSI cable, buy the highest quality and _shortest_ cable you  
can get your hands on.  NeXTConnection has 18" cables (though I am not sure how  
high quality they are --- they're thin).  If you have a lot of devices, take  
minimal length over absolute top quality (my current approach, not definitive  
--- I am still having problems).

The DAT probably works on the "end" of the chain because you've shortened the  
chain by removing the HSD Color Scanner.  I've gone through this very process  
(and am about to do so again, since my DAT seems to be acting up :-( ).

My (current) comprehensive SCSI checklist follows:

1.  your SCSI id.s --- they shouldn't (and don't look like they) conflict.   
There should be no duplicate numbers.  The numbers should range from 0-7.  On  
the NeXT, I believe 7 is reserved for the CPU or somesuch (floppy?), leaving  
0-6 for other devices.  Also, on the NeXT, 1 is used for internal drives, I  
believe, leaving 0,2-6.  You probably want to leave 0 vacant to be able to  
insert a temporary SCSI device as a boot device (for emergencies, testing,  
whatever).  So, available SCSI id.s are 2-6 (only 5 free ones :-().
2.  your SCSI termination on all devices (you already have, probably, but  
double-check).  You need to terminate the first and last (physically, i.e.,  
where the cable ends --- not (necessarily) the lowest/highest SCSI id.) devices  
on the SCSI chain.  The first device is the internal drive, so it should  
already be terminated.
3.  your SCSI cables --- that they are good quality.  Try swapping cables to  
determine if any are faulty, even just repositioning cables in the chain can  
help.
4.  your SCSI connections --- reseat your SCSI cables to ensure the connections  
are solid and haven't loosened.
5.  your external SCSI cable length (let me guess: 5 devices, 4 cables for  
roughly 4x4' = 16'?)  6m = about 18' maximum.  You need less than 18' total  
(sum of internal and external SCSI cables).  Each of your SCSI devices has some  
SCSI (ribbon, usually) cable inside the box to connect the external connector  
to the internal device.  This cable can be around 3' long in a dual SCSI box,  
so you should double-check your internal SCSI cable lengths for "hidden" SCSI  
cable.
6.  (mythical option)  get an "active SCSI terminator"!  When I consulted the  
oracle (Usenet) early on in my SCSI problems, two people suggested getting one  
of these mythical beasts.  I have tried half a dozen electronics stores in the  
SF Bay Area to no avail (and have been told no such thing exists several  
times).  If anyone knows what this is or how to get one, please let me know!   
It's supposed to help SCSI chain quality problems and, presumably, eliminates  
problems on border length SCSI chains (that's what I want one for anyway! :-).

Unfortunately, even a sub-18' SCSI chain is not guaranteed to work (as I am  
finding out :-().  Needless to say, I have become quite un-enamored with SCSI  
bus design.  Flakey by design and lacks diagnostics to verify sources of  
problems --- can you say *bad* *bad* *bad*.  (I knew you could. :-)

I hope these empirical tidbits help.  If anybody has corrections, new insights  
or suggestions, please let me know too (or just post!).

Thanks,

= Joe =
-- 
Joe Grace <JGrace@TetraSoft.com> (NeXTMail preferred.)
TetraSoft International   Business and Personal Productivity Tools for NeXTSTEP
Newsgroups: comp.sys.next.misc