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Terminating the SCSI Bus

August 24th, 2009

Terminating The SCSI Bus What is Termination?

Cables make up the physical connections of the SCSI bus. Since the SCSI bus is a chain of devices on a cable rather than a loop, the two ends of the bus must be terminated. Every wire in the cable has a specific impedance, or resistance to the passing of electrical signals. When signals reach the end of the cable that makes up the SCSI bus, they encounter the air, which has very high impedance and acts as a wall of infinite resistance. The problem with high impedance is that any signal coming down the bus is reflected back in the other direction once it hits this barrier. Terminating both ends of the cable prevents the signal from being reflected.
You terminate the bus by attaching a circuit, the terminator, to the physical ends of the SCSI bus. The terminator provides an impedance that matches the cable’s, thereby preventing the signal from bouncing back. The terminators use power, and the power to operate them comes from the SCSI interface card through the termination power wire on the bus.

What is Passive Termination?

Passive Termination is the oldest method of termination, defined in the specs for SCSI-1. Basically, a passive terminator sits on the bus to minimize reflections at the end of the cable. The terminator doesn’t really do any work to regulate power for termination; it relies on the interface card to provide steady power. A passive terminator simply provides impedance that’s close to the impedance of the cable.

What is Active Termination?

Active termination works to control the impedance at the end of the SCSI bus by using a voltage regulator, not just the power supplied by the interface card. Because it is active, regulating the power that it gets from the interface card, active termination is more stable than passive termination.

What is Forced Perfect Termination (FPT)?

Forced perfect termination is the most complex of the terminators, going beyond merely stabilizing the power applied to the terminator. It can actually alter its impedance to compensate for variations in impedance among many different cables, devices, and terminators. It is usually used in high-speed SCSI systems that have many different devices, cables, and terminator types. The complexity of such a system can cause impedance mismatches that degrade the signals sent through the bus. FPT actively compensates for these impedance variations by means of diode switching and biasing to force the impedance of the cable to match each device.

How do I terminate Seagate SCSI disc drive?

If you are installing a Seagate drive in a system that has other SCSI devices installed, terminate only the end devices on the SCSI chain. A SCSI “device” is any disc drive, scanner, tape backup unit, or other piece of hardware connected to your system using the SCSI bus.

Terminating The SCSI BUS

The example above shows an internal hard disc at one end of the SCSI bus with the SCSI controller at the other end (both are terminated). The bottom example shows two additional SCSI devices connected externally-this means the SCSI controller is no longer on the end of the SCSI chain and should not be terminated.

Note: Some controllers prefer to remain terminated even if they are in the middle of the chain. Also, some controllers treat the internal and external chains as separate logical buses. This means you may need to terminate both the first and last devices on both logical buses to achieve proper termination. If necessary, refer to your system or controller documentation to see how this is handled in your particular system.

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