The Future of SCSI
SCSI continues as the device of choice for systems in which speed and compatibility are important. The ability of the SCSI format to provide fast and efficient fault tolerance for network systems through the use of RAID (redundant array of independent disks) will keep it as the drive of choice for networks. Although it is not required, the SCSI drive is generally preferred over IDE by Windows NT system designers for its performance and flexibility. SCSI continues to be more expensive than IDE, but SCSI's ability for RAID, hot plugging (changing drives without shutting down a system), and machine independence will keep it popular for workstations and servers.
Setting Up a SCSI Subsystem
There are several steps in setting up a SCSI-based system or adding a new SCSI peripheral to an existing system. Performing these steps in the proper order, without shortcuts, is the key to a fast, easy installation.
Start with the Host Adapter
SCSI cards come in a wide variety of sizes, shapes, and configurations. Some offer one connection, others have four. Options include secondary or even tertiary channels-RAID, cache RAM, and so forth. Be sure that the card will be able to service the devices planned for it. Set any jumpers first, then install the SCSI adapter card in the appropriate expansion slot.
Set the SCSI IDs, Termination, and Peripheral Cabling
Write down the ID for each device-including the host adapter-as it is assigned. After the IDs are set, verify termination for each end of the chain. Finally, attach the cables-first to the host adapter, then to the closest internal device-and move outward on the chain. Repeat the process for the external devices.
External devices usually use some form of switch to set the ID. Most allow setting IDs from 0 through 7 only. You might need to adjust that with internal devices that often allow a wider range of ID numbers. Cable types include: 50-pin Centronics type, SCSI-2 D-Shell 50, and 68-p type connectors. Make sure the last device in the chain is properly terminated.
Internal SCSI devices are installed inside the computer and are connected to the host adapter through an interior connector on the host adapter. Check the connection diagram to be sure the fitting is the right one for that type of device. The options are a 50-pin ribbon cable (similar to a 40-pin IDE cable) and two similar 68-pin cables. Be sure to use the right type of 68-pin cable: one is for ultra-low voltage differential (LVD), and the other is for single-ended (SE) drives. They are NOT interchangeable.
IMPORTANT
SCSI devices connected incorrectly (for instance, with the cable plugged backwards) can be damaged! Be sure the red or blue strip on the cable is facing toward pin 1. Some SCSI devices allow only a proper connection.
Power Up One Device at a Time
A good practice is to connect the power to one device, power up, and check for problems. Then power additional devices one at a time and make sure everything is working and without conflict.
Load Operating System Drivers and SCSI Software
Finally, load any software required to allow the operating system to recognize the new hardware and take full advantage of its features.
Using a cable with enough connectors enables you to easily link multiple internal devices. You can have up to eight (numbered 0 through 7) devices, or 16 (numbered 0 through 15, depending on the host adapter and the devices) on a single SCSI chain. Don't forget that one position in each SCSI chain is taken up by the host adapter. Figure 9.3 shows a SCSI chain.
Figure 9.3 SCSI Chain
The exact number of devices will vary depending on a number of conditions. The host adapter must support the number selected, the installer must be able to set proper IDs, and the cables and connectors must be compatible. Older adapters allow only seven total IDs, and the card will use one, leaving you with six devices. Some SCSI devices have limited ID options. Many older products have only seven possible settings; some scanners or optical products are factory-set to an ID. Given the range of cable options and performance considerations, you might have to limit the number of devices on a single chain to get maximum performance.