SCSI Installation Tips. Black Box IC501A

IC500A 7/12/00 12:38 PM Page 30

SCSI FIBER OPTIC BUS EXTENDERS by varying manufacturers’ peripheral devices. SCSI terminators compensate for these inherent impedance mismatches on a SCSI bus where peripheral devices such as hard drives, CD-ROM drives, scanners, or printers are used.

Passive Terminators

The most basic is a passive-resistance style terminator. This is usually supplied with peripherals and frequently does a poor job of balancing the impedance of the

SCSI bus. Passive terminators are resistor networks that allow signal voltages to vary with the load and terminator power supplied, resulting in unstable signals from end to end on the bus and causing data errors. Passive terminators are no longer recommended by ANSI for designs.

Active Terminators

Active terminators add a voltage regulator to the circuit to regulate signal voltages with varying loads and termpower, allowing a consistent signal to be transmitted everywhere on the bus which compensates for the varying bus lengths and signal loads. Active termination is the minimum ANSI-recommended termination.

7.2 SCSI Installation Tips

Keep your SCSI chain short. Official SCSI specifications limit a SCSI chain to no more that 19.7 feet (6 meters) long. Practical experience says the shorter the better. The maximum length you should allow between devices is 3 feet (0.9 m).

Never assign the same SCSI ID number to two devices on the same bus. SCSI uses these numbers as addresses to ensure that information goes to the correct location.

Giving two devices the same address can result in lost information.

Know that some SCSI-ID numbers may be reassigned. Internal boot hard drives are usually set to ID 0, while secondary hard drives are set to 1. Motherboards or host adapters are generally set to ID 7.

Always terminate the first and last devices on the chain. Drives purchased specifically for internal use nearly always arrive with terminators installed. If in doubt, call the vendor you purchased a device from.

If the last device on the chain has two SCSI connectors, attach the cable to one and a

terminator to the other. Otherwise you’ll have an open connector that may cause noise on the SCSI chain.

Always turn off the power to your computer and SCSI devices before swapping cables or

moving devices around. SCSI cables contain sensitive data-transmission lines and one or more live power wires.

30