Conventional SCSI signaling is very similar to that used for most other interfaces and
buses within the PC. Conventional logic is used: a positive voltage is a "one",
and a zero voltage (ground) is a "zero". This is called single-ended signaling, abbreviated SE. Up until
recently, single-ended SCSI had been by far the most popular signaling type in the PC
world, for a simple reason: it is relatively simple and inexpensive to implement.
There's an important problem with SE signaling, however. SCSI is a high-speed bus
capable of supporting multiple devices, including devices connected both inside and
outside the PC. As with all high-speed parallel buses, there is always a concern about
signal integrity on the bus; problems can arise due to bouncing signals, interference,
degradation over distance and cross-talk from adjacent signals. The faster the bus runs,
the more these problems manifest themselves; the longer the cable, the more the problems
exist for any given interface speed. As a result, the length of a single-ended SCSI cable
is rather limited, and the faster the bus runs, the shorter the maximum allowable cable
length.
To get around this problem, a different signaling method was also defined for SCSI,
which uses two wires for each signal that are mirror images of each other. For a logical
"zero", zero voltage is sent on both wires. For a logical "one", the
first wire of each signal pair contains a positive voltage, similar to the signal on an SE
bus, but not necessarily at the same voltage. The second wire contains the electrical
opposite of the first wire. The circuitry at the receiving device takes the difference
between the two signals sent, and thus sees a relatively high voltage for a one, and a
zero voltage for a zero. This method is much more resilient to signaling problems
than regular SE signaling. It is called differential signaling, after the
technique used to determine the value of each signal by the recipient. The two signals in
each pair are usually named with "+" and "-" signs; for example, the
signal carrying data bit 0 would use "+DB(0)" and "-DB(0)". See the section cabling for more information.
This table shows the great difference in cable length that exists between SE and
differential devices, particularly as bus speed increases:
Signaling Speed |
Bus Speed (MHz) |
Single-Ended SCSI
Maximum Cable Length (m) |
Differential SCSI
Maximum Cable Length (m) |
Slow |
5 |
6 |
25 |
Fast |
10 |
3 |
25 |
Fast-20 |
20 |
1.5 |
25 |
As you can see, each doubling of the bus speed results in a halving of the maximum
cable length for single-ended SCSI, but differential SCSI allows long (25m) cables for all
three speeds. (Fast-20 buses allow a cable length of 3m if no more than four devices are
used, but this is really a kludge of sorts to get around the limitations associated with a
1.5m cable restriction.)
Differential SCSI is a great idea in theory, and one might have thought it would become
very popular. In fact, this never happened in the PC world, largely due to cost. The
circuits needed to drive differential signals are more expensive and use more power than
those for single-ended SCSI. For many years, single-ended SCSI was "good
enough", and allowed cable lengths sufficient for the needs of most users, so little
impetus was seen to move to the more expensive differential signaling. From there,
"chicken-and-egg" syndrome kicked in: since differential was less popular, it
was not produced in volume and so never saw its costs come down due to economies of scale.
The end result of all of this is that the older type of differential signaling is
rarely seen in the PC world. The concept of differential signaling, however, did not die
out. As the SCSI bus was pushed to faster and faster speeds, the cable limits of SE were
finally too great to be worked around. However, the cost of regular differential was
unappealing, so a new type of differential signaling was created, called low voltage
differential or LVD. See this page for more on LVD
signaling. With the creation of LVD, the old name of "differential" for the
higher-voltage version became vague, so the older style was renamed high voltage
differential or HVD.
High voltage differential signaling has been around since the earliest SCSI-1 standard, so devices have been theoretically available as
either SE or HVD since the start of SCSI use on the PC--of course, whether any particular
model has been implemented in HVD is another story altogether. 
With the creation of
the SPI-3 specification and the standardization of LVD,
HVD had no more raison d'etre, and has been removed from the SCSI standard
entirely, leaving only LVD. There are no new drives being produced that use HVD.
Note: Since differential
signaling uses complementary positive and negative voltages, it is sometimes called balanced
signaling; single-ended signaling is similarly called unbalanced. This is just
different terminology for the same technologies.
Warning: Since
single-ended and HVD SCSI use very different voltage levels, they are incompatible at the
electrical level. You should not mix single-ended (or low voltage
differential) devices with high voltage differential SCSI devices on the same bus. If
you do, actual physical damage could result--this is one of those cases where actually
smoking your hardware is a distinct possibility, because of the high voltages that might
be sent to the single-ended or LVD devices! To compound the matter, the cables and
connectors used for single-ended and differential SCSI look the same.
To help reduce the chances that similar-looking SE and HVD hardware will be
interconnected, special icons are imprinted on SCSI hardware that indicates the
signaling method used by the device. Make sure you know what you have before putting
together your SCSI bus, and look for these identifying symbols on devices to be sure they
are electrically compatible. Note that slightly different symbols are used for LVD and
LVD/SE devices, as described here. Also
see this section for more on cables and connectors.
Next: Low-Voltage Differential (LVD) Signaling
