[wplug] Re: SCSI compatibility -- SCSI disconnect, LVD v. SE signaling, etc...

Bryan J. Smith b.j.smith at ieee.org
Fri Jun 29 14:53:55 EDT 2007


[ Okay, so much for lurking ... I'm too much of a hardware fiend. ]

curlynoodle at gmail.com wrote:
> Is it possible to run a SCSI Ultra320 drive on a Ultra160 controller 
> accepting the obvious reduction in performance?

Michael Skowvron wrote:
> Yes. They are backward compatible and will negotiate. You could even run 
> your U320 drive on an old 10MB/sec narrow SCSI-1 channel if you needed to.

As Michael pointed out, Ultra160 (aka Ultra3) and Ultra320 (aka Ultra4)
are compatible.  These two "Ultra" speeds are also compatible with prior
"Ultra" and "Fast" SCSI, as they use the same voltage, although with
caveats.

Without going too deep, I want to point out four things ...

1.  SCSI-2 protocol and device disconnect/negotiation
2.  Single Ended (SE) versus Low-Voltage Differential (LVD)
3.  Maximum Bus lengths of SE versus LVD
4.  Know about High Voltage Differential (HVD), and how to ID it

1.  SCSI-2 protocol and device disconnect/negotiation

The SCSI-2 protocol supports device disconnects.  So if you have an
multiple devices of different speeds, assuming they use the same
electrical channeling (see the next bullet on SE and LVD), the fastest
data rate will be used that is possible between the devices.

E.g., an Ultra320 card with one Ultra160 disk and one Ultra320 disk will
speak 160MBps between the Ultra160 disk and either the controller or
other Ultra320 disk, and 320MBps when the Ultra320 controller and
Ultra320 drive are communicating.  Any device incapable of using the
DTR, as long as the channel signaling is the same, will disconnect or
otherwise not be involved with the communication.

Note that controller setup and other driver settings can change this,
and possibly avoid the added bus context overhead (long story).  It all
depends on the devices involved and tuning.

2.  Single Ended (SE) versus Low-Voltage Differential (LVD)

Ultra80 (Ultra2), Ultra160 (3), Ultra320 (4) and the proposed Ultra640
(5) are Low-Voltage Differential (LVD) SCSI.

LVD uses the exact same cabling as earlier Ultra40 (UltraWide), Fast20
(FastWide) and Wide[10] SCSI, 16-bit data, except LVD uses a second set
of wires for differential.  [ Yes, this also means that SE SCSI
terminators do _not_ work for LVD SCSI terminators, although vice-versa
suffices. ]

When you have all LVD devices on the SCSI bus, the maximum speed is
whatever the controller is.  I.e., at least Ultra80 (Ultra2).

When you have one SE device on the SCSI bus, the maximum speed is 40MBps
(Ultra40/UltraWide).  This is because differential signaling, which
requires the second set of conductors, cannot be utilized by all drives.

So other than avoiding using older, slower SE devices on a LVD bus when
other LVD devices are present, you can safely mix'n match any LVD device
speeds on most SCSI controllers.  This includes your case.

3.  Maximum Bus lengths of SE versus LVD

Differential channeling allows much greater lengths.  That's another
reason, besides speed, to keep your LVD channel completely LVD.  Once
you introduce SE, you _cut_ the maximum channel length in addition to
speed.

   SE:   6m (Wide10), 3m (Fast20) and 1.5m (Ultra40)
  LVD:  12m (all, at least through Ultra320) or 25m single device

4.  Know about High Voltage Differential (HVD), and how to ID it

The only other thing you should be concerned with is High Voltage
Differential (HVD).  *NEVER* use HVD unless you have all HVD equipment.
SE and LVD are compatible, HVD is _not_ -- including the cabling.  Even
if people don't try the electronics, I've seen some "well burned" SCSI
cables and terminators when used with HVD when they were not.  ;)

Know the symbols for SE and LVD versus HVD.  LVD has the less than sign
(<) _inside_ the diamond.  HVD have the less than sign (<) _outside_ the
diamond.  SE is just the diamond.  LVD "fits in the SE diamond," HVD
does not.  ;)

Plug these abbreviations into Google images to find example
illustrations.  For the most part every LVD supports SE, although there
is a technical reservation that you can have a LVD without SE support
(no "dash/tab" on the right of the diamond with a < inside).

BTW, there _are_ HVD implementations beyond Ultra40, although they are
rare (typically non-existent after Ultra80).  They are largely required
where noise (EFI) and other considerations would make LVD undesirable.

-- Bryan 'yes, I like to hear myself speak' Smith  ;->

TANGENT:  Of course, all of this "parallel SCSI" is made much more
simple with Serial Attached SCSI (SAS) -- the SCSI-2 protocol over,
essentially, the simple, point-to-point Serial ATA compatible
interconnect.  SAS goes up to 8m (and can be considered a "same closet
SAN" for multi-target SAS hubs/devices).  It's not SATA (which only goes
1m, possibly 2m shielded/eSATA), and requires twisted pair cabling
(although 6GHz SATA-III will as well), but you can use SATA drives on
SAS cards (with its limitations).


-- 
Bryan J. Smith         Professional, Technical Annoyance
mailto:b.j.smith at ieee.org   http://thebs413.blogspot.com
--------------------------------------------------------
        Fission Power:  An Inconvenient Solution



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