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SCSI Bus Termination

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.

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

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