UNIX System Administration Handbook - Evi Nemeth [80]
Internal SCSI devices are usually attached to a ribbon cable; only one port is needed on the actual SCSI device because connectors can be clamped onto the middle of the ribbon cable. When using a ribbon cable, make sure pin 1 on the SCSI bus is connected to pin 1 on the hard drive.3
Each end of the SCSI bus must have a terminating resistor (“terminator”). These resistors absorb signals as they reach the end of the bus and prevent noise from reflecting back onto the bus. Terminators take several forms, from small external plugs that you snap onto a regular port to sets of tiny resistor packs that install onto a device’s circuit boards. Some devices are even autoterminating.
SCSI-1 used a different terminator design from that of later versions of SCSI (“passive” rather than “active”), so very old terminators (and old self-terminating devices) can cause problems on a newer SCSI bus. In practice, we have rarely found such old terminators to be an issue.
One end of the bus normally terminates inside the host computer, either on the SCSI controller or on an internal SCSI drive. The other end usually terminates on an external device, or on the SCSI controller if there are no external devices. If you experience seemingly random hardware problems on your SCSI bus, first check that both ends of the bus are properly terminated. Improper termination is one of the most common SCSI configuration mistakes, and the errors it produces can be obscure and intermittent.
Each device has a SCSI address or “target number” that distinguishes it from the other devices on the bus. Target numbers start at 0 and go up to 7 or 15, depending on whether the bus is narrow or wide. The SCSI controller itself counts as a device and is usually target 7 (even on a wide bus, for backward compatibility). All other devices must have their target numbers set to unique values. It is a common error to forget that the SCSI controller has a target number and to set a device to the same target number as the controller.
A SCSI address is essentially arbitrary. Technically, it determines the device’s priority on the bus, but in practice the exact priorities don’t make much difference. Some systems pick the disk with the lowest target number to be the default boot disk, and some require the boot disk to be target 0.
If you’re lucky, a device will have an external thumbwheel with which the target number can be set. Other common ways of setting the target number are DIP switches and jumpers. If it is not obvious how to set the target number on a device, consult the hardware manual. Most hardware specifications can be found on the manufacturer’s web site these days; trying to set up a random disk used to involve quite a lot of trial and error.
The SCSI standard supports a form of subaddressing called a “logical unit number.” Each target can have several logical units inside it. A plausible example might be a drive array with several disks but only one SCSI controller. However, logical units are seldom used in real life. When you hear “SCSI unit number,” you should assume that it is really a target number that’s being discussed until proven otherwise. If a SCSI device contains only one logical unit, the LUN usually defaults to 0.
SCSI buses are generally quite easy to configure, but a variety of things can go wrong:
• Many workstations have internal SCSI devices. Check the listing of current devices before you reboot to add a new device. Remember that most tape systems and some floppy drives (most notably, HP’s) are SCSI.
• Make sure that a differential SCSI controller has only differential devices and differential terminators connected to it, and make sure that a single-ended