You can do an experiment (either physically or mentally) to illustrate why termination
is required on a SCSI bus. Hold one end of a piece of rope about six feet long and have
someone else hold the other end. Stretch the string so it is reasonably taut, but not
tight, and then snap down on one end sharply. You will form a wave that travels down the
string. When it reaches the end of the string it will "reflect" off the end and
travel back again toward you, and then reflect again. It will go back and forth across the
string, decreasing in amplitude each time until it eventually dies out.
Electrical signals travel across wires in much the same way as physical waves travel
across a string. When they reach the end of the wire, they will reflect and travel back
across the wire. The problem is that if this is allowed to happen, the reflected signals
will interfere with the "real" data on the bus and cause signal loss and data
corruption. To ensure that this does not happen, each end of the SCSI bus is terminated.
Special components are used that make the bus appear electrically as if it is
infinite in length. Any signals sent along the bus appear to go to all devices and then
disappear, with no reflections.
There are several different kinds of termination used on SCSI buses. They differ in the
electrical circuitry that is used to terminate the bus. Better forms of termination make
for more reliable SCSI chains; the better the termination, the fewer problems (all else
being equal) with the bus, though cost is generally higher as well. In general terms,
slower buses are less particular about the kind of termination used, while faster ones
have more demanding requirements. In addition, buses using differential signaling (either
HVD or LVD) require special termination.
Here are the different types of SCSI termination:
- Passive Termination: This is the oldest, simplest and least reliable type of
termination. It uses simple resistors to terminate the bus, similar to the way terminators
are used on coaxial Ethernet networks. Passive termination is fine for short, low-speed
single-ended SCSI-1 buses but is not suitable for any modern
SCSI speeds; it is rarely used today.
- Active Termination: Adding voltage regulators to the resistors used in passive
termination allows for more reliable and consistent termination of the bus. Active
termination is the minimum required for any of the faster-speed single-ended SCSI buses.
- Forced Perfect Termination (FPT): This is a more advanced form of active
termination, where diode clamps are added to the circuitry to force the termination to the
correct voltage. This virtually eliminates any signal reflections or other problems and
provides for the best form of termination of a single-ended SCSI bus.
- High Voltage Differential (HVD): Buses using high
voltage differential signaling require the use of special HVD terminators.
- Low Voltage Differential (LVD): Newer buses using low voltage differential signaling also require their own special
type of terminators. In addition, there are special LVD/SE terminators designed for use
with multimode LVD devices that can function in either LVD or SE modes; when the bus is
running single-ended these behave like active terminators.
|
Internal (above) and external active terminators.
The LED on the external shows that the terminator is connected.
Some multimode LVD/SE terminators have LEDs that light up
one color when the bus is running in LVD mode, and a different
color when it is running in SE mode. This is useful for troubleshooting. |
Original images © Computer Cable Makers, Inc.
Images used with permission. |
Terminators must be at the very ends of the bus, after all of the actual devices on the
chain. This includes any devices that may be powered off or temporarily disconnected.
Therefore, there are always exactly two terminators per bus or bus segment. Many devices
contain internal terminators that can be used if the device is at one of the ends of the
SCSI bus. However, differential drives typically do not include the ability to terminate
the bus, so newer LVD applications require explicit terminator hardware. Sometimes
terminators are built in to the end of the SCSI cable. In addition, systems using the Single Connector Attachment system have a different
termination arrangement because the connection system is different. SCA drives do not have
termination on them.
Note: Host adapters
usually do include the ability to terminate the SCSI bus. In fact, many host adapters
include multiple segments, and so have the ability to
terminate each segment they support. Termination should only be enabled on a host adapter
if the host adapter is the last device on any segment. If you are using both internal and
external devices on a host adapter that has only one logical segment being shared by both
internal and external drives, the host adapter is going to typically be in the middle of
the chain between them, and its internal termination should be disabled.
Termination is a rather straight-forward affair when all of the devices on the SCSI bus
are the same width: either narrow (regular, 8 bit) or wide (16 bit) SCSI. When you mix
narrow and wide SCSI on the same bus, you must be more careful about termination. The
issue that arises is that if part of the device is running in wide mode, but not all
devices are wide, half of the data lines (the "high byte") may end somewhere on
the bus; they need to be terminated, and that termination may occur in a different place
than where the "low byte" data signals are terminated.
Normally these issues are handled using special adapters or cables that only extend the
extra width to the devices that are using the wide portion of the bus. However, the extra
signals on the wide part of the bus must also be terminated properly. Problems
can result with wide devices when these extra signals are not terminated and are left
"dangling". See this discussion of mixing wide and
narrow devices for more details.
Next: Summary of SCSI Cables and Connectors
