No, SE is not LVD, (error made late at night.)  However, controllers that
handle LVD are usually able to handle SE, everything becomes SE.  Some places
are now equating SE and LVD, or at least using them together.

I went to the Adaptec FAQ to fish out the following information:

"SCSI-1/2 (5MB/Sec) 6 meters (18ft)
SCSI-2 fast (10MB/Sec) 3 meters (9 ft)
SCSI-2 fast/wide, ULTRA (20MB/Sec) 3 meters (9 ft)
SCSI-2 ULTRA Wide (40MB/Sec) 1.5 meters (>4 devices)
SCSI-2 ULTRA Wide (40MB/Sec) 3 meters (<4 devices)
SCSI-3 ULTRA2 Narrow (40MB/Sec) 12 meters (>2 devices)
SCSI-3 ULTRA2 Wide (80MB/Sec) 12 meters (>2 devices)

*(DPB Note there is now ULTRA 160 (160MB/Sec))*

Q: I was told not to use 'Differential' drives on my host adapter. Why? What is
a differential drive?
A: SCSI devices use 'single-ended' or 'differential' electrical interfaces
between the device and host adapter. A single-ended interface uses one wire for
each data or command/status line, with a corresponding ground wire for that
signal. Voltage is carried only on the signal line.

A differential interface uses signal transmitters and receivers to drive the
signal over longer distances and for noise rejection. The signal is split into
positive and negative components and transmitted over cable pairs. Voltage is
carried on both wires. The signal is re-combined at the receiving device. If a
single-ended device, such as a CD-ROM, is connected to a differential host
adapter (e.g., AHA-2944UW), or a differential device (e.g., a hard drive with a
D suffix)is connected to a single-ended host adapter, such as the AHA-2940UW,
*irreparable damage can be done to either or both devices*. Adapters are
available from third-party suppliers which adapt differential devices to
single-ended uses.

Low Voltage Differential (LVD) devices are classed as ULTRA2 and should be used
only on adapters which support ULTRA2 devices (The AHA-2940U2W supports both
LVD and SE devices)."


Emphasis in the data above is mine.  Connecting Differential and SE devices on
the same bus is the IT version of Russian Roulette.  Most times nothing will
happen, but every once in a while. . .

Kind regards,

Denys. . .

Denys Beauchemin
HICOMP
(800) 323-8863  (281) 288-7438         Fax: (281) 355-6879
denys at hicomp.com                             www.hicomp.com


-----Original Message-----
From:   Steve Dirickson [SMTP:[log in to unmask]]
Sent:   Saturday, May 20, 2000 3:58 AM
To:     [log in to unmask]
Subject:        Re: DDS-4 and SCSI

>  When one talks about FWSCSI on MPE, one is really talking
> about Fast Wide
> Differential (AKA HVD High Voltage Differential) SCSI, as
> opposed to Fast Wide
> Single-ended (AKA LVD or Low Voltage Differential)SCSI.

SE is LVD???

> All this to say that if your DDS-4 drive is a Differential
> SCSI drive, you
> should be able to connect it to the FWSCSI.  If it is not
> differential, DO NOT
> CONNECT IT, its controller will get fried.

Actually, it won't. Differential (HVD) devices automatically disable
their line drivers if they sense ground on the DIFFSENS line, which is
grounded by SE devices (or, perhaps more properly, DIFFSENS is used as
an active-high enable). Mixing SE and HVD (and LVD, for that matter)
on a bus won't hurt anything; the HVD devices will disable themselves,
and the LVD devices will operate in SE mode.


Steve Dirickson   WestWin Consulting
[log in to unmask]   (360) 598-6111