Figure 926 in Software

Generator QR Code JIS X 0510 in Software Figure 926

External SATA (eSATA) extends the SATA bus to external devices, as the name would imply The eSATA drives use similar connectors to internal SATA, but they re keyed differently so you can t mistake one for the other Figure 927 shows eSATA connectors on the back of a motherboard eSATA uses shielded cable lengths up to 2 meters outside the PC and is hot pluggable The beauty of eSATA is that it extends the SATA bus at full speed, so you re not limited to the meager 50 or 60 MBps of FireWire or USB

eSATA connectors (center; that s a FireWire port on the left)

Many specialized server machines and enthusiasts systems use the small computer system interface (SCSI) technologies for various pieces of core hardware and peripherals, from hard drives to printers to high-end tape backup machines SCSI is different from ATA in that SCSI devices connect together in a string of devices called a chain Each device in the chain gets a SCSI ID to distinguish it from other devices on the chain Last, the ends of a SCSI chain must be terminated Let s dive into SCSI now, and see how SCSI chains, SCSI IDs, and termination all work SCSI is an old technology dating from the late 1970s, but it has been continually updated SCSI is faster than ATA (though the gap is closing fast), and until SATA arrived SCSI was the only good choice for anyone using RAID (see the RAID section a little later) SCSI is arguably fading away, but it still deserves some mention

SCSI manifests itself through a SCSI chain , a series of SCSI devices working together through a host adapter The host adapter provides the interface between the SCSI chain and the PC Figure 928 shows a typical PCI host adapter Many techs refer to the host adapter as the SCSI controller, so you should be comfortable with both terms

All SCSI devices can be divided into two groups: internal and external Internal SCSI devices are attached inside the PC and connect to the host adapter through the latter s internal connector Figure 929 shows an internal SCSI device, in this case a CD-ROM drive External devices hook to the external connector of the host adapter Figure 930 is an example of an external SCSI device Internal SCSI devices connect to the host adapter with a 68-pin ribbon cable (Figure 931) This flat, flexible cable functions precisely like a PATA cable Many external devices connect to the host adapter with a 50-pin high density (HD) connector Figure 932 shows a host adapter external port Higher end SCSI devices use a 68-pin high density (HD) connector Figure 929 Internal SCSI CD-ROM Multiple internal devices can be connected together simply by using a cable with enough connectors Figure 933, for example, shows a cable that can take up to four SCSI devices, including the host adapter Assuming the SCSI host adapter has a standard external port (some controllers don t have external connections at all), plugging in an external SCSI device is as simple as running a cable from device to controller The external SCSI connectors are D-shaped, so you can t plug them in backward As an added bonus, some external SCSI devices have two ports, one to connect to the host adapter and a second to connect to another SCSI device The process of connecting a device directly to another device is called daisy-chaining

You can daisy-chain up to 15 devices to one host adapter SCSI chains can be internal, external, or both (see Figure 934)

If you re going to connect a number of devices on the same SCSI chain, you must provide some way for the host adapter to tell one device from another To differentiate devices, SCSI uses a unique identifier called the SCSI ID The SCSI ID number can range from 0 to 15 SCSI IDs are similar to many other Figure 932 50-pin HD port on SCSI host adapter PC hardware settings in that a SCSI device can theoretically have any SCSI ID, as long as that ID is not already taken by another device connected to the same host adapter Some conventions should be followed when setting SCSI IDs Typically, Old SCSI equipment allowed most people set the host adapter to 7 or 15, but you can change this setting SCSI IDs from 0 to 7 only Note that there is no order for the use of SCSI IDs It does not matter which device gets which number, and you can skip numbers Restrictions on IDs apply only within a single chain Two devices can have the same ID, in other words, as long as they are on different chains (Figure 935) Every SCSI device has some method of setting its SCSI ID The trick is to figure out how as you re holding the device in your hand A SCSI device may use jumpers, dip switches, or even tiny dials; every new SCSI device is a new adventure as you try to determine how to set its SCSI ID