What is the difference between USB and scsi?

Answer 1

In it's day, SCSI was a great, high-speed disk interface. Today, IDE has pretty much taken over and SCSI isn't really worth the effort that it takes to support it. However, SCSI does have an option for an external connection.

USB is an acronym for Universal Serial Bus and it's a great way to interconnect devices with your computer.

Each USB device is SUPPOSED to have a built in identification code that allows the computer to identify what it's talking to. Drivers are available, either built in to Windows, Apple or Linux systems, downloadable across the internet or included with the device when you buy it.

Most USB devices are truly "universal".

Answer 2

THE SCSI INTERFACE

For many years, the SCSI (Small Computer Systems Interface) has been the standard interface used for connecting devices to your Mac. The benefits of using this interface standard include its having been widely used for many years, so most types of hardware are available with a built-in SCSI port. Also, over the years the SCSI interface has evolved, and it now is available in several configurations including ultra high speed up to 160 MB/sec (megabytes per second).

The downside to using SCSI-based devices include that this interface can be highly temperamental (i.e., devices refuse to mount, disk errors occur, strange system problems occur, etc.), no more than 7-15 devices can be connected together in a single SCSI chain, you must assign each device a distinct SCSI ID number before adding it to your SCSI chain, the last device in the chain must be terminated, and all the SCSI devices in the chain must be turned off (along with your computer) before a SCSI device may be added to or removed from the SCSI chain (i.e., SCSI doesn't support plug and play/hot swapping of devices). Additionally, you must be careful to turn on all your external SCSI devices (and, when appropriate, allow them to spin up to speed) before turning your computer on, and you must remember to shutdown your computer before turning these devices off. Also, you need to use thick, relatively expensive, and heavily shielded cables when connecting SCSI devices to each other and to your Mac.

Basically, the SCSI interface offers both a large selection of devices to choose from and is capable of high speed performance, but it can be a real pain to work with.

THE USB INTERFACE

Several years ago, Apple decided to add a USB (Universal Serial Bus) port to the Mac. The benefits of using this interface include your not having to assign a unique ID number to each of your USB devices (this is done for you by your computer), there is no need to terminate the last device in the USB chain, each USB port can support up to 127 devices, and the USB interface supports hot swapping (USB devices may be added and removed while your computer is turned on).

You ask, "Is there a downside to using USB-based devices?" Of course there is. ;-) While in theory USB devices should support hot swapping, after connecting some USB devices to your Mac, they won't work until you restart your computer. Also, some USB hard drives can't be used as a startup volume, and some USB devices won't work properly unless they are connected directly to a USB port on your computer. (In theory, you should be able to plug a USB device into a multi-port USB hub which expands the number of USB ports that are available to you.) Perhaps the biggest downside is speed or, more specifically, the lack thereof. The current USB interface supports a maximum speed of 12 Mbps (megabits per second), and this is only a small fraction of the speed that is supported by both the SCSI and FireWire interfaces.

Also, each device that is on a USB chain shares the bandwidth (data carrying capacity) for the port on the Mac that it's connected to. Consequently, when several USB devices are attempting to simultaneously transfer data, they must share the already limited capacity of the USB interface (this can dramatically slow down these devices and your Mac). For this reason, in the real world, it's not likely most users would want to have many USB devices connected to these low-capacity USB ports.

Basically, most USB hardware like printers, scanners, and storage devices are convenient to use, but they are relatively slow when compared to their SCSI and FireWire counterparts.

To make things a little more interesting (maybe I should say confusing), there is a USB 2.0 interface that may be appearing very soon, and it will support a data transfer rate of up to 480 Mbps. However, even though in theory it may be faster than FireWire, it has yet to prove itself, Apple hasn't yet agreed to support it, and there presently are virtually no devices that work with it (some USB 2.0 compatible devices are scheduled to ship this Spring).

SPECIAL NOTE: You should be careful when comparing performance specifications between these interfaces. While both SCSI and FireWire performance are often stated in MB/sec (megabytes per second), USB performance is commonly stated in Mbps (megabits per second).

Since there are 8 bits (a bit is the smallest unit of information that your computer works with) per byte, the USB rating of 12 Mbps translates to only 1.5 MB/sec, so you can see USB performance is far below that of either SCSI (up to 160 MB/sec) or FireWire (up to 50 MB/sec).

Also, while "port" and "bus" are often used interchangeably, the port is the place on your computer where you connect a cable to the bus (the "roadway" your data travels on in your computer). Bandwidth indicates the information carrying capacity of this "roadway." The interface (SCSI, FireWire and USB) determines the "rules of the road" (the electronic specifications that control how your data is handled).