[rescue] two SCSI oddities

[George Adkins]

On Thursday 29 November 2001 01:10 am, you wrote:
> I have a stack of Seagate Elite 9 drives that I'm in the process of
> verifying -- I'm one drive away from filling up my 12-bay SCSI
> expansion pedestal :). Two questions have come up that I'm sure have
> easy answers, but I'm getting nothing out of google:
>
> 1. I couldn't get the bus to sync at 10MB/sec until I, scratching
>    head, removed the "terminator enable" jumper on the drives. It
>    would only sync at 5 with that jumper in. What's the electrical
>    reason for this?

Over termination,

trippple termination pulls the bus currend down so low, that the system wil 
only auto-negotiate to 5M/s if at all.

once at the HBA, once at the drice, and once at the end of the bus.

the term - enable on the drive is there to use if you mount it internal on a 
ribbon cable with no terminator on the end.

external enclosures , mount all the drives with the termination off and put 
an external terminator on one of the external connectors.

This quirk (5M/sec being sooo tolerant) is why SS2's and IPX'en can get away 
with people putting terminated drives in them (sometimes), they also should 
have no terminators on internal drives, as the termination for the internal 
bus is on the motherboard, and the drive is actually mounted on a stub off 
the main bus.  If you terminate the internal drive though, it can play hell 
with external devices if attached.

George