🚀 go-pugleaf

Thanks to all who replied, as requeste here is a summary of replies.


In article <1676@aurora.AthabascaU.CA> you write:
>
>
>
>After spending a couple hours trying to cable up some SCSI peripherals
>to a SUN i would like to pose the following questions.
>
>A SCSI bus consists of a 50 pin bus running from a SCSI controller.
>Bus length can be up to 15 feet.
>On this bus you can have up to 8 devices.
>Connectors for getting on the bus come in many different flavors,
>(Centronics style, AMP-socket connectors, D style connector)
>Some SCSI peripherals provide a bus extention to allow the bus to 
>continue on to the next device. This is simply a parallel extention of the bus.
>
>so far so good?
yes.
>
>Where must a SCSI bus be terminated? Only once, and at the last device on 
>the bus would seem to be the logical choise.
Actually twice - at both ends of the bus (read transmission line).
>
>When taping into a SCSI bus, the tap or stub lenght can only be a few
>inches in lenght i guess? But can you get away with a longer stub lenght
>if you also terminate that specific device?
NO.  You would get nasty reflections no matter what you did.  As it turns
out SCSI isn't horribly fast so you might get away with it, but random 
errors may show up.

===============================================================================

>Where must a SCSI bus be terminated? Only once, and at the last device on 
>the bus would seem to be the logical choise.

	Twice - both ends of the bus. The host doesn't have to be at
	an end, either, although this is very common.

>When taping into a SCSI bus, the tap or stub lenght can only be a few
>inches in lenght i guess? But can you get away with a longer stub lenght
>if you also terminate that specific device?

	I've seen stubs have problems at about a foot, and work at
	about 5 feet. It all depends on the devices in use. Best advice,
	don't risk your data - do it right.  Having an extra terminator
	is definitely out - will cause lots of problems.

	Watch out for who is providing terminator power - you can easily
	blow a fuse (many devices provide fuses expicitly for this) which
	will cause stange and mysterious problems. 
===============================================================================

> Where must a SCSI bus be terminated? Only once, and at the last device on 
> the bus would seem to be the logical choise.

No, twice - at each end.  But the host adapter is usually at one end, and
it is usually terminated at the factory, so you don't need to fool with it.

> When taping into a SCSI bus, the tap or stub lenght can only be a few
> inches in lenght i guess? But can you get away with a longer stub lenght
> if you also terminate that specific device?

The standard lists 6.0 meters max, single-ended SCSI bus, with a stub length
of 0.2 meters, max, at any device.  The reason you can't have a stub of any
length that is terminated is that you basically end up with two infinite
(looking) lines coming out of the branch, and the signal strength is then
halved at the branch.  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.)
===============================================================================

You must terminate an SCSI bus at both ends (first device and last device).  No other termination is allowed or necessary.  The 15-foot length is for single-ended SCSI only; differential allows longer.

===============================================================================

so far not so good.
maximum spec-ed length is 6m.
the bus passes through devices.  no parallel connections are allowed
although i think a couple of short stubs within a cabinet is within
the spec.  
===============================================================================
> Bus lenght can be up to 15 feet.

I believe the spec allows 6 meters for nondifferential SCSI, and 25
meters for differential SCSI.

> On this bus you can have up to 8 devices.

But your host adapter counts as one of the 8.

> Where must a SCSI bus be terminated? Only once, and at the last device on 
> the bus would seem to be the logical choise.

Close.  I believe it is supposed to be terminated on each endpoint of
the cable.  Since one endpoint is likely to be inside your host, it may
already be terminated.

> When taping into a SCSI bus, the tap or stub lenght can only be a few
> inches in lenght i guess? But can you get away with a longer stub lenght
> if you also terminate that specific device?

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

Adaptec used to print a good book on SCSI.  Maybe they still sell them
or give them away.  It has been 4-5 years since I got my copy.

===============================================================================
My books say 18 ft. is okay.  Only the last device may
be terminated.
===============================================================================
SCSI host adapters typically terminate their end internally.  You have to worry
about the other end.  The official length limit for "single-ended" SCSI (this
refers to the bus drivers, the alternative is "differential") is 6 meters.
The maximum stub length is 3 inches.

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.
===============================================================================
Your assumptions are pretty much on the money.

SCSI busses are to be terminated at each end of the Bus.  SCSI host adapters
(Your 3/50 for example, or a VME SCSI card or whatever) have terminators 
built in.  You should also terminate the last device.

As for the stubbs, if you look inside a SCSI shoebox (with both in and
out connections) you will see that there is a ribbon cable thet looks
from one to the other.  An inline 50 connector is cripmed onto the middle of 
the cable, for each device in the showbox, and plugs into the device.  
There is no stub except for the SCSI device PCB itself.

.reality on

In reality you can abuse things quite badly and SCSI will still work.  It
is very forgiving.  You can usually leave one of the terminators off and
not get burnt.  I have also seen really scary cabling setup where they
built a star type cable out of ribbon cables.  It worked, but I would hate
to depend on it.  The reflections must be terrible.  If you are having 
difficulty seeing a device try another cable and make sure things are 
addressed properly.  Unless you're doing something really ridiculous
your cabling scheme is likely not the problem.

.reality off

===========================================================================
1. The maximum length of a single-ended (non-differential) SCSI bus is 6 Meters.

2. Termination must be at both ends of the bus, and nowhere else.
   Typically, the bus is terminated at the HBA on on end, and at the last device or
   with an external terminator at the other end.

3. Maximum stub length is 0.1 Meters.

4. Maximum number of devices on a bus is 8, incuding the Host Bus Adaptor (HBA).

5. Many people violate some of the above, and the bus still works. You will find 
   less chance of a non-conformant bus working as you add devices and increase the
   speed (i.e. synchronous operation). Some devices are more sensitive to bus
   variations than others.

===========================================================================
Twice... once at each end.  If your host (PC, Mac, whatever) is at
one end of the bus, it should have a terminator.  If you have a very
short bus (e.g. a Mac and a single hard disk connected via a 1' cable),
a single terminator at the disk may suffice.

> When taping into a SCSI bus, the tap or stub lenght can only be a few
> inches in lenght i guess?

The SCSI-1 spec says ".1 meter", or roughly four inches.

>                       But can you get away with a longer stub lenght
> if you also terminate that specific device?

Adding a third terminator to the bus will probably cause it to cease
functioning.  The terminators do more than terminate the bus with the
correct impedence... they also act as pull-up circuitry, holding the
bus (in its idle state) at roughly 3 volts.  If you have too many
terminators on the bus, a SCSI device may not not be able to sink enough
current to pull the bus down to the low-voltage state... and it may
thus be unable to transmit properly.

You may be able to get away with a longer stub if your bus is not heavily
populated, and/or your SCSI devices are robust, and/or you're not
pushing the bus to its limits with high-speed synchronous data transfers.

You may be able to _eliminate_ a stub entirely.  If your SCSI box has
a pair of connectors ("SCSI IN" and "SCSI OUT", for example), you
can wire the jacks together with a horseshoe-shaped loop of ribbon
cable, long enough to reach over to the SCSI device(s) in the box.
This way, the loop inside the box becomes part of the SCSI bus,
rather than being a stub.

> confirmation/correction of the above would be greatly appreciatted, and
> any other cabling/hardware tips also welcome.

Hacking SCSI setups is a bit of a black art.  There's a lot of science
to it... but it seems that the cut&try method is still necessary a
lot of the time.
===========================================================================
	UPTO 20 feet and that includes the bus lenght used *inside* the 
	enclosure of choice.

>On this bus you can have up to 8 devices.
	Actually only 7.  The 8th (target 7) is the host adapter itself.

>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.

>Some SCSI peripherals provide a bus extention to allow the bus to 
>continue on to the next device. This is simply a parallel extention of the bus.
	This is actually a continuation of the bus.

>Where must a SCSI bus be terminated? Only once, and at the last device on 
>the bus would seem to be the logical choise.
	Yes.  It is the last physical device not the highest id's device.

>When taping into a SCSI bus, the tap or stub lenght can only be a few
>inches in lenght i guess?
           (LENGTH) This stub can only be 5inches long.

>But can you get away with a longer stub lenght (LENGTH)
>if you also terminate that specific device?
	No.

	1. If you want your system to run reliabably don't exceed the 
	20foot limit.
	2. Besure all your connections are screwed down.
	3. Do not bend those thick cables too tight or you'll pull the 
	pins out.
	4. Don't connect or reconnect devices while power is on.
	
	Failure to follow the above points is asking for hardware failure
	and/or scrambling the image on the disk.
===========================================================================
>A SCSI bus consists of a 50 pin bus running from a SCSI controller.

	yes, though there is a more-or-less standard 25-pin version (25+
of the normal 50 are ground).

>Bus lenght can be up to 15 feet.

	Actually 6 meters.

>On this bus you can have up to 8 devices.

	That includes the host, remember.

>Some SCSI peripherals provide a bus extention to allow the bus to 
>continue on to the next device. This is simply a parallel extention of the bus.

	Sort of.  There are sub-addresses which can be dealt with in any
way the device wants to (Logical Unit Numbers (LUNs), ranging from 0-7).
Most devices implement only one LUN (0).

>Where must a SCSI bus be terminated? Only once, and at the last device on 
>the bus would seem to be the logical choise.

	Normally yes (though I think the host adapter may need termination,
I'm a software guy so I'm not sure without checking).

>When taping into a SCSI bus, the tap or stub lenght can only be a few
>inches in lenght i guess? But can you get away with a longer stub lenght
>if you also terminate that specific device?

=========================================================================
No.  A SCSI bus must be terminated at each end of the cable.  If the cable
starts inside a computer and, via an external connector, exits to the outside
world and connects to a device, then the terminators must be removed off
the adapter and the bus would be terminated at both ends of the cable.

>When taping into a SCSI bus, the tap or stub lenght can only be a few
>inches in lenght i guess? But can you get away with a longer stub lenght
>if you also terminate that specific device?

A stub is not a good idea.  This leaves you open to all kinds of unreliable
operations.  If you have a controller that supports a number of LUN's, then
termination of those LUN's are handled independent of the SCSI bus.  The
controller itself is the only main concern.
=========================================================================