🚀 go-pugleaf

> The standard lists 6.0 meters max, single-ended SCSI bus, with a stub length
> of 0.2 meters, max, at any device.  

Only .1 meters for single-ended.  The .2 meter stub length is when
differential transceivers are used.

> By limiting the length of the stub, and not terminating it, then the
> reflection comes back "immediately" to add to the rest of the signal going
> down the main transmission line.  (This explanation may have a large bullshit
> factor.  Please post or email a summary of what you get.)

No BS - this is essentially correct.  A rising signal being propagated down
the bus will indeed be attenuated at device stubs, in proportion to the
reflection coefficient produced by the stub in parallel with the main bus.  An
80 ohm main bus in parallel with an 80 ohm stub will appear as a 40 ohm
discontinuity in the cable.  Some of the initial signal will be reflected back
to the source, and another portion will continue on down the main bus.  The
*magnitude* of the "glitch" is proportional to the relative impedances.  The
*duration* of the glitch is a function of the stub length.  The longer the
length, the more likely a device will see a low-going pulse where none was
intended.

> My guess is that long taps are not allowed, even if you terminate the taps.  

Please don't "terminate" these taps!  As noted, only two termination circuits
are permitted, one at each end of the bus.  Adding termination circuits may
damage the drivers on any of the attached devices. It may also degrade the
low-level noise margin by increasing V(ol) beyond the 0.5V maximum.

> Most SCSI boxes I've seen go to the trouble of routing the cable
> in to the SCSI devices, then back to the outbound connector.

Short of using special "Y" cables, having two connectors is the only way to
daisy chain the cables used on drives with shielded bulkheads.

> SCSI host adapters typically terminate their end internally.  You have to 
> worry about the other end.  

In general, but not always.  For example, some host adapters are actually in
the "middle" of the bus - peripherals internal to the host cabinet are at the
end.  The connector you see may have to be terminated if there are internal
but no external peripherals, or if there are external but no internal
peripherals.  If there are both, the host adapter would have to be
"unterminated".

> You can usually get away with a lot of cheating if your cables are
> short.  Missing termination at one end or an extra termination in the
> middle is often not fatal.  Slightly longer stubs are probably ok.
> If you must cheat, make sure SCSI bus parity checking is enabled
> everywhere.

In general, the bus doesn't become a transmission line until the propagation
delay exceeds the rise/fall time of the signals it carries.  At transition
times of 5ns and cable speeds of about 1.5-2ns/ft, this means the cable had
better be no greater than about 24".

> In reality you can abuse things quite badly and SCSI will still work.  It
> is very forgiving.  

I've found the opposite to be true, when using single-ended systems with more
than 4 devices on cable lengths longer than 4 meters.

> You can usually leave one of the terminators off and not get burnt.  

But please don't try it!

> Unless you're doing something really ridiculous your cabling scheme is 
> likely not the problem.

I've found cabling and termination to be the primary source of problems
on maximally configured systems. Use high-impedance cables, and Alternative-2
termination!


> UPTO 20 feet and that includes the bus lenght used *inside* the 
> enclosure of choice.

Good point - some devices use up to a meter or two for internal cable routing.

>>Connectors for getting on the bus come in many different flavors,
>	There are only two that are actually true standard: the champ
>	or Centronics style, and the DB-50 style.  Sun's new tiny thing on
>	the SPARCStations is their own at this point.

There also are new "high-density" connectors with side latches instead of bail
lock clips defined in SCSI-2. They are much smaller than the Champ connector
and may be what you're seeing on Sun's products.

> (LENGTH) This stub can only be 5inches long.

0.1 meters = 3.937"