That’s the best explanation of “WHY” you can’t have a slave only disk on
IDE. Hell, even I understood it.
Regarding the unterminated SCSI, yes, I was essentially kidding. But
everyone knows that in a very quiet system (shhhh, the computer is sleeping)
if you only try to do one IO at a time an unterminated SCSI bus will give
the illusion of working.
So you get a few standing waves. Hello, ain’t that what makes PCI so fast?
Maybe we shouldn’t terminate SCSI buses. The terminators probably just slow
all those signals down.
SAFETY NOTICE: DO NOT TRY THIS AT HOME! Bill Caroselli is a known wise ass
and was JUST KIDDING!
<firstname.lastname@example.org> wrote in message news:email@example.com…
Well why shouldn’t it work? My SCSI disks seam to work even though
not properly terminated. ;~}
I bet you’re just kidding, but facts for others:
PC/AT HDD system was originally an Western Digital WD1003 HDD Controller
sitting on I/O 1F0-1F7,3F6-3F7 IRQ 14, controlling two Seagate ST-506
(Note: not two controller two HDD; ONE controller two HDD,
just like ONE FDD controller (uPD765) and two FDD)
ISA — WD1003 ± ST506
Now when IDE was introduced, the WD1003 moved onto the HDD pcboard
and “IDE Interface” became merly ISA bus with an address decoder
When you connect TWO IDE disks, TWO former WD1003 controller will
now sit onto the same I/O and IRQ.
That’ll cause electrical/logical conflict.
ISA --(buffer)± [(former WD1003)–HDD]
± [(former WD1003)–HDD]
So you have to tell one of the controller to act as “slave mode” and
only respond upon commands/stats/whatever which only affects the second
Other one will be assigned “master” and responds to commands which
affects the controller itself (common to two HDDs) and things only for
If you now disconnect the “master” and leave “slave” dangling, IDE
which affects both drives will be unanswered, which will cause
something unpleasant (hang etc)
SCSI bus is designed as a LAN, so every node can work for itself.
Lacking termination will only cause signal degradation and making
cables short will make non-terminated SCSI bus work sometimes.