Differential signaling has been around since the first SCSI specification, SCSI-1. As described in detail on this
page, high voltage differential (HVD) signaling uses two wires for each signal to
improve signal integrity and allow long cables to be used without data loss or corruption.
However, it is expensive to implement, and uses a great deal of power. For this reason, it
has never caught on, and through the late 1990s, conventional single-ended (SE) signaling
was the standard in the SCSI world.
Single-ended SCSI signaling worked fine for many years. However, when the SCSI industry
was ready to increase the speed of the SCSI bus to 40 MHz, they had a serious problem. As the table on this page shows, each doubling of the bus speed
results in a halving of the maximum cable length allowed under SE. Since 20 MHz speeds had
already dropped maximum cable length to 1.5m, halving it again would have resulted in a
maximum length of only 0.75m--that's just a little over two feet for an entire SCSI chain!
The alternative was to go to the older type of (high voltage) differential signaling, with
its high cost and catastrophic electrical incompatibility with single-ended hardware.
Instead, a third option was created, with the intention of marrying the best attributes
of both SE and HVD signaling. This is a differential signaling method that was designed to
use the advantages of differential signaling to allow long cable lengths, while reducing
implementation cost and allowing for electrical compatibility with single-ended devices.
This technology is called low voltage differential or LVD signaling. It
was first defined in the SPI-2 standard and is rapidly
becoming the signaling method of choice in the SCSI world. In fact, LVD signaling is
required for Ultra2 or Wide
Ultra2 SCSI (unless HVD is used), and LVD is the exclusive signaling method for all
SCSI modes faster than Ultra2. Even the fastest LVD SCSI chains can be up to 12m in
length, or 25m if only two devices are used on the chain (this is called point-to-point
operation; remember that one of these must be the interface card, the host
adapter.)
The concept behind LVD is relatively straight-forward: continue using two wires for
each signal, but use lower voltage to create the complementary signal pairs. Using lower
voltage allows cost to be reduced and power requirements to be kept under control. It also
means that the dangers associated with mixing SE and differential devices is eliminated.
In fact, single-ended devices are not just electrically compatible with LVD devices, some
types of LVD devices can even function on single-ended SCSI buses.
A particular type of LVD device was defined when LVD was created; drives that
correspond to this variant of LVD are called multimode LVD device. These are
usually abbreviated LVD/SE or LVD/MSE (the "M" is for
"multimode"). A multimode LVD device will automatically switch between LVD and
single-ended operation by detecting whether the other devices on the chain are running in
SE or LVD mode. (Note that only one or the other can be used at a time; the device won't
use both simultaneously.)
In addition to the usual SCSI rules--such as unique IDs for
each device and proper topology and termination--LVD operation requires the following:
- All devices on the chain must be LVD-capable; if even one device is only SE, all devices
"drop down" and run as single-ended.
- All devices must not be set to run in SE mode; some multimode devices have a jumper to "force" SE operation, which will
cause the entire SCSI chain to not work in LVD.
- LVD (or multimode LVD/SE) terminators must be used.
Remember that bus speeds over 20 MHz are not supported under single-ended operation.
This means that a multimode LVD/MSE Ultra160 device will run at only a maximum of 40 MB/s
if it is connected to a SCSI chain with single-ended devices.
Warning: As soon as
multimode LVD devices begin running as single-ended, all the rules and restrictions of
single-ended operation apply, including cable length. For example, suppose you have a 4m
cable connecting an LVD Ultra160 host adapter to a multimode LVD Ultra160 device; this is
perfectly fine. Now, let's say you decide to add to this cable a Wide Ultra single-ended
device. As soon as this happens, the other devices will drop down to single-ended
operation, and probably will try to run at Ultra speeds (Fast-20). Communication problems
will then result due to the fact that a 4m cable is not supported at Ultra speeds in
single-ended operation.
LVD signaling is rapidly taking over the SCSI world. Single-ended operation is not
supported for bus speeds faster than 20 MHz ("Ultra"
bus speeds), so to use Ultra2 or faster SCSI, differential must be used. HVD for its part
was made obsolete in the SPI-3 standard, so for Ultra3, Ultra160, Ultra160+ and faster speeds, LVD is the only option.
Warning: Low voltage
differential devices are not electrically compatible with high voltage differential
hardware; do not mix them on the same SCSI cable or damage to the LVD devices may occur.
To help reduce the chances that similar-looking LVD and HVD hardware will be
interconnected, special icons are imprinted on SCSI hardware that indicates the
signaling method used by each 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
SE and HVD devices, as described here. Also
see this section for more on cables and connectors.
Next: SCSI Bus Width
