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Termination

All buses need to be electrically terminated at each end; the SCSI bus is no exception. Improper termination still is one of the most common problems in SCSI installations. Three types of terminators typically are available for the SCSI bus:

  Passive
  Active (also called Alternative 2)
  Forced Perfect Termination (FPT): FPT-3, FPT-18, and FPT-27

Typical passive terminators (a network of resistors) allow signal fluctuations in relation to the terminator power signal on the bus. Usually, passive terminating resistors suffice over short distances, such as 2 or 3 feet, but for longer distances, active termination is a real advantage. Active termination is required with Fast SCSI.

An active terminator actually has one or more voltage regulators to produce the termination voltage, rather than resistor voltage dividers. This arrangement helps ensure that the SCSI signals always are terminated to the correct voltage level. Active terminators will usually have some sort of LED indicating the termination activity. The SCSI-2 specification recommends active termination on both ends of the bus and requires active termination whenever Fast or Wide SCSI devices are used. Most high-performance host adapters have an "auto-termination" feature so if it is the end of a chain, it will terminate itself.

A variation on active termination is available: Forced Perfect Termination. Forced Perfect Termination is an even better form of active termination, in which diode clamps are added to eliminate signal overshoot and undershoot. The trick is that instead of clamping to +5 and Ground, these terminators clamp to the output of two regulated voltages. This arrangement enables the clamping diodes to eliminate signal overshoot and undershoot, especially at higher signaling speeds and over longer distances.

FPT terminators are available in several versions. FPT-3 and FPT-18 versions are available for 8-bit standard SCSI, while the FPT-27 is available for 16-bit (Wide) SCSI. The FPT-3 version forces perfect the three most highly active SCSI signals on the 8-bit SCSI bus, while the FPT-18 forces perfect all the SCSI signals on the 8-bit bus except grounds. FPT-27 also forces perfect all of the 16-bit Wide SCSI signals except grounds.


Note:
Several companies make high-quality terminators for the SCSI bus, including Aeronics and the Data Mate division of Methode. Both of these companies make a variety of terminators, but Aeronics is well-noted for some unique FPT versions that are especially suited to problem configurations that require longer cable runs or higher signal integrity. One of the best investments that you can make in any SCSI installation is in high-quality cables and terminators.

SCSI Drive Configuration

SCSI drives are not too difficult to configure, especially compared with IDE drives. The SCSI standard controls the way that the drives must be set up. You need to set two or three items when you configure an SCSI drive:

  SCSI ID setting (0 – 7)
  Terminating resistors

The SCSI ID setting is very simple. Up to eight SCSI devices can be used on a single SCSI bus, and each device must have a unique SCSI ID address. The host adapter takes one address, so the rest are free for up to seven SCSI peripherals. Most SCSI host adapters are factory-set to ID 7, which is the highest-priority ID. All other devices must have unique IDs that do not conflict with one another. Some host adapters boot only from a hard disk set to a specific ID. In my system, for example, the IBM SCSI host adapter requires the boot drive to be set to ID 6. Newer IBM host adapters and systems enable you to boot from a hard disk at any SCSI ID. Older Adaptec host adapters required the boot hard disk to be ID 0; newer ones can boot from any ID.

Setting the ID usually involves changing jumpers on the drive itself. If the drive is installed in an external chassis, the chassis may have an ID selector switch that is accessible at the rear. This selector makes ID selection a simple matter of pressing a button or rotating a wheel until the desired ID number appears. If no external selector is present, you must open the external device chassis and set the ID via the jumpers on the drive.

Three jumpers are required to set the SCSI ID; the particular ID selected actually is derived from the binary representation of the jumpers themselves. For example, setting all three ID jumpers off results in a binary number of 000b, which translates to an ID of 0. A binary setting of 001b equals ID 1, 010b equals 2, 011b equals 3, and so on. (Notice that as I list these values, I append a lowercase b to indicate binary numbers.)

Unfortunately, the jumpers can appear either forward or backward on the drive, depending on how the manufacturer set them up. To keep things simple, I have recorded all the different ID jumper settings in the following tables. Table 2.9 shows the settings for drives that order the jumpers with the Most Significant Bit (MSB) to the left; Table 2.10 shows the settings for drives that have the jumpers ordered so that the MSB is to the right.

Table 2.9  SCSI ID Jumper Settings with the Most Significant Bit to the Left.

SCSI ID Jumper Settings

0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1

1 = Jumper On, 0 = Jumper Off

 
Table 2.10  SCSI ID Jumper Settings with the Most Significant Bit to the Right.

SCSI ID Jumper Settings

0 0 0 0
1 1 0 0
2 0 1 0
3 1 1 0
4 0 0 1
5 1 0 1
6 0 1 1
7 1 1 1

1 = Jumper On, 0 = Jumper Off
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