[SunRescue] SCSI: Active vs. Passive termination in SPARCstations 2, LX

G W Adkins rescue at sunhelp.org
Tue Jun 5 11:32:41 CDT 2001

>  Glancing through "the book of SCSI" at the local Barnes and Nobles, I
> read across an interesting chapter on termination that seemed to propagate
> that 1) active termination is better as it keeps the bus quieter 2) active
> and passive can be mixed (?!!) for performance..
>  Not that I'm about to go out and buy an active terminator to try it out
> myself, I'm wondering the merits of such information, as well as if one
> should avoid this 'book of SCSI' with great effort.

No way!  That book is a fantastic sourcebook for those uninitiated to SCSI.
Top Notch.  I highly recommend it.
As far as the Active/Passive thing, on a short bus a single active
terminator or forced perfect terminator paired with a passive unit is just
fine.  At less than 20Mhz/18 inches, you should have no problems.
(heck, in Sun4c's like the IPX and SS2, you can get away with just the
internal terminator on the motherboard if there is no external cabling
attached.  the whole bus is so short, that a single passive terminator does
the trick at 5 Mhz.  Kinda bending the rules, but it works just fine.)

>  That said- by default, is the SCSI host adapter (even on above mentioned
> machines) have termination disabled? (since it would be in the middle of a
> SCSI chain of internal and external devices) and is there a way to
> toggle/determine the current setting of such a thing?

on the older sun gear, the host adatper is on the motherboard with
termination at the far end, after the plug-ins for the drives.  the drives
are mounted on short ribbons, (the SCSI Spec allows stubs of up to 4 inches)
so you avoid the "Y" problem.)

ext. conn. ------ SCSI Host Adapter ------ HDD conn. ---- HDD conn. ---
passive term.

>  I'm going to be upgrading the fleet this summer, and one of my goals is
> to tweak as much as possible, including my tower of 411 cases + LX :)
>  Thanks.
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