[rescue] Wide disk on Narrow bus, reprise

Curious George jorge234q at yahoo.com
Mon Feb 11 11:29:30 CST 2008

Hi, Mark,

--- Mark <md.benson at gmail.com> wrote:

> On 9 Feb 2008, at 17:39, Curious George wrote:
> > --- Mark <md.benson at gmail.com> wrote:
> >
> >> mostly in Macs and Amigas. In my experience, you
> >> need to get hold of 2
> >> things - a terminator for the end of the 50 pin
> >> ribbon (and a ribbon
> >
> > Why does the ribbon need an extra terminator?
> Wide drives don't have onboard terminators. 50-pin
> busses must be end terminated

Yes, but note I attached a wide terminator to the
end of the wide bus (at the wide drive).  The narrow
side of the bus exists only on the LX's motherboard
(where, presumably, it is terminated at the SCSI

The 8 low order data lines plus associated control
signals *for* the 50 pin bus extend onto my "wide"
cable and, thus, are terminated at the drive end.

Also, the "high order" data lines -- that only
connect to the wide drive -- are terminated by
that same terminator.  Note that the "host end"
of those lines are unterminated -- but, the SCSI
protocol says they shouldn't be used unless a
wide negotiation has taken place (and that
negotiation takes place entirely on the "narrow"

> (68-pin ones seem to work both ways in my
> experience  
> however, possibly they work LVD terminated and/or SE
> unterminated??).  
> I know that all the issues I've ever had getting
> wide drives working  
> that weren't down to dodgy Dell/Compaq firmware were
> usually to do  
> with not having terminator. You can substitute the
> terminator for a 50- 
> pin device with one onboard (how they would be setup
> from the factory)  
> but as the LX (AFAIK) ha
> >> That's my 2p worth.
> >
> > It's my understanding that the drive should
> request
> > wide transfers (negotiation) and, if the host
> > declines, fall back to narrow.  I.e., I don't see
> > anything *wrong* with the approach I have taken...
> I think the upper byte terminators in the adapters I
> have force the  
> drives down to Narrow mode by default, because the
> upper btye is  
> terminated at the back of the drive, and so no
> signal is passed  

"Termination" doesn't mean "end".  Rather, it
means there is a load on the cable to match the
characteristic impedance of the cable, etc.
I.e., in my case, the wide cable "ends" at the
LX's motherboard (specifically, *in* the 50
pin connector on that motherboard since my
"adapters" don't pass those signals through)

> through it. I do have some adapters that aren't
> upper byte terminated,  
> come to think of it, that seem to work in most
> circumstances, but if I  
> know the machine is picky I'll use the upper byte
> terminated ones.
> > I'll have to dig through the Standard to see how
> > much of this is "optional" vs. mandated.
> The biggest issue is SCSI is a non-standard
> standard. Controllers,  
> cabling, etc. vary so much between machines it's a
> black art getting  
> some of them to work. Sun stuff seems pretty hardy
> SCSI-wise. I've  
> hardly had any problems that weren't due to PEBCAK
> :).

(sigh)  I'll set it aside and dick with it when I
have some time to spare...


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