> Of course it leads to erratic behaviour. If you have no termination
> what so ever on the SCSI bus, then the Adaptec cards will refuse to
> operate at all. The reason is that no termination results in all
> kinds of open circuit where they should be closed and grounded. The
> hardware ends up reading all kinds of nasty and unintended values
> from the SCSI bus when there is no termination. Poor termination is
> simply a precursor to this complete fail state :)
I'm not sure that you understand what termination is.
If you have a cable, and hoist one end of it from 0V to 5V in a few
ns, the other end will take a while before it notices that you did
that (speed of light at least). Worst case the other end will have a
completely different wave form, and you will have trouble deducing what
the other end did to generate this. However with a small trick, you
can make the wave form travel intact along the wire (the "transmission
line effect"): you have to make the conductor be surrounded by a
cylindrical grounded plane. Coax. This effect is reasonably strong: A
grounded wire on both sides of the conductor in a flat cable is already
a reasonable approximation of the coax leading to the transmission
line effect.
The trouble starts when such a conductor abruptly ends. At that point,
the travelling wave form simply bounces back and starts propagating
back to where it came from. However with current performance
requirements, the source will be wanting to send the next bit along
the wire by then. This will lead to data corruption. To prevent this
the transmission line will have to be lead to believe that there is no
abrupt ending to the cable. This turns out to be relatively easy: a
simple resistor to a DC level will do. With SCSI termination, they
have one more trick: The DC level has been chosen such that without
any drivers the signal level will be somewhere around the switch point
of the then-common TTL chips. This requires someone to actively pull
it down to get a reliable "0", or to actively pull it up to make a
reliable "1". This distributes the burden of transmitting data over
both the pull-down and the pull-up output transistor, instead of only
requiring just one. (You also get faster circuits if you use both of
them.)
Coming back to what you said, "open" versus "closed and grounded" does
not make sense to me. If there would be "current driven" drivers
around, that might make sense, but a SCSI driver simply needs to drive
the signal level to 0V or 5V. Without termination however you get
interference from the bounce-back of the travelling wave front.
Roger.
-- ** R.E.Wolff@BitWizard.nl ** +31-15-2137555 ** http://www.BitWizard.nl/ ** Florida -- A 39 year old construction worker woke up this morning when a 109-car freight train drove over him. According to the police the man was drunk. The man himself claims he slipped while walking the dog. 080897