SCSI Interfaces

The Small Computer System Interface (SCSI) is the best interface for scanners under Linux. Most scanners use the 8-bit (25- or 50-pin) Narrow SCSI format. Modern SCSI scanners usually support at least Fast SCSI-2, although they don't need the 10MB/s speed of which Fast SCSI-2 is capable. (A 24-bit, 1,200-dpi scan of an 8.5x11-inch page consumes 385MB, and so could be passed over a 10MB/s SCSI connection in 38.5 seconds. This is faster than most scanner mechanisms can produce an image of this size.) Even if scanners don't normally saturate a SCSI bus, the SCSI bus's speed is an improvement over the 1.5MB/s of USB or the 2MB/s of the parallel port. When a manufacturer offers the same basic mechanism in both SCSI and parallel or USB varieties, the SCSI version is typically about twice as fast as the other versions.

Most SCSI scanners use 8-bit, 25-pin interfaces. Most modern SCSI adapters, though, provide only a single 68-pin external connector. This combination can make interfacing the scanner to modern SCSI host adapters difficult, because 68-pin-to-25-pin SCSI cables are extremely rare. The task can be accomplished with a 68-pin-to-50-pin cable and a 50-pin-to-25-pin adapter, such as the one in Figure 19.3.

Figure 19.3

Adapters can be very helpful in linking mismatched SCSI components.

Figure 19.3

Adapters can be very helpful in linking mismatched SCSI components.

Chapter 19

Another option is to use a second, less-powerful SCSI host adapter for the scanner than you use for Wide SCSI hard disks. (This idea also works with some other non-disk devices.) In fact, many SCSI scanners come with SCSI host adapters, although these adapters are usually very low-end, and might produce slower performance than you can get from even inexpensive Fast SCSI-2 host adapters. Some bundled SCSI adapters are also unsupported in Linux. Of course, if you don't use SCSI hard disks, you probably don't need a powerful Wide adapter, so a SCSI card that supports only Narrow devices is just fine.

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