| What do they do?
The Small Computer Systems Interface (SCSI) and FireWire (IEEE 1394) are high-speed systems for connecting peripherals. SCSI, the older, is a cross-platform standard with support for hard disks and other storage media, scanners, specialist printers, and others. FireWire, a recent international standard, has fewer devices, but includes hard disks and storage media, digital video systems, and more in the future. How do they work?
SCSI buses are daisy-chains of devices with different identity numbers. The large connectors and thick cables allow many data lines, along which devices transfer data and commands to one another. Variants support speed enhancements, including ‘wide’ data transfer. FireWire uses just three twisted-pair cables. FireWire devices are connected in chains, and mustn’t form loops, but this standard can support 63 as opposed to just seven devices at once, and is more flexible. Data can be transferred at speeds of 100Mb/sec or more, and future enhancements should reach 1Gb/sec, but hard disks are currently much slower. SCSI devices should be used by turning the peripherals on first, then your Mac, and reversing the sequence when shutting down. FireWire peripherals can be connected and disconnected while your Mac is still running (hot-swapping). What can go wrong with them?
Although most SCSI hard disks are well-behaved, scanners and other devices can be tricky to get working reliably. Troubleshooting a sick SCSI chain is a black art, and may involve swapping positions, adding or removing termination, and more. No two SCSI devices on the same bus may have the same ID number. FireWire is much simpler at present, although this may change as it becomes more popular. While hot-swapping is neat, you must drag a FireWire volume to the Trash before disconnecting it or turning it off, or your Mac may freeze. How can I upgrade them?
Most peripheral devices need software drivers, which should be kept up to date through Web support sites. Macs using PCI cards for their SCSI interface can have the card swapped for a faster one, but using slower devices on the same bus can make them all run slowly. PCI cards adding FireWire are also available, and you need to keep Apple’s FireWire extensions up to date. Further info
Web sites worth visiting include Granite Digital’s excellent SCSI troubleshooting guide at www.scsipro.com/trbshoot/02fs_trouble.htm, and Michael Amorose’s FireWire Watch, tracking FireWire products at www.michael-amorose.com/firewire. | |
