FireWire

FireWire or iLink (IEEE designation 1394) is a personal computer and digital video serial bus interface standard offering high-speed communications and isochronous real-time data services, developed primarily by Apple Inc., completing development in 1995. It is defined in IEEE standard 1394 which is currently a composite of three documents: the original IEEE Std 1394-1995, the IEEE Std 1394a-2000 amendment, and the IEEE Std 1394b-2002 amendment. Sony's implementation of the system is known as i.Link, and uses only the four signal pins, discarding the two pins that provide power to the device in favor of a separate power connector on Sony's i.Link products.

The system is commonly used for connection of data storage devices and digital video cameras, but is also popular in industrial systems for machine vision and professional audio systems. It is used instead of the more common USB due to its faster speed, higher power distribution capabilities, and because it does not need a computer host. It also has native support for isochronous data transport (data that must be delivered with deterministic latency, such as audio or video). However, the small royalty that Apple Inc. and other patent holders have initially demanded from users of FireWire ($0.25 per end-user system) and the more expensive hardware needed to implement it ($1-$2) has prevented FireWire from displacing USB in low-end mass-market computer peripherals where cost of product is a major constraint.

Features
It can connect together up to 63 peripherals in an acyclic network structure (as opposed to SCSI's linear structure). It allows peer-to-peer device communication, such as communication between a scanner and a printer, to take place without using system memory or the CPU. Firewire also supports multiple hosts per bus, and through software IP networks can be formed between Firewire-linked computers. It is designed to support plug-and-play and hot swapping. Its six-wire cable is not only more convenient than SCSI cables but can supply up to 45 watts of power per port, allowing moderate-consumption devices to operate without a separate power cord. (Note that the Sony-inspired iLink usually deletes the power part of the cable/connector system and only uses a 4-pin connector.)

FireWire 400 can transfer data between devices at 100, 200, or 400 Mbit/s data rates (actually 98.304, 196.608, or 393.216 Mbit/s, but commonly referred to as S100, S200, and S400). Cable length is limited to 4.5 metres but up to 16 cables can be daisy-chained yielding a total length of 72 meters under the specification.

FireWire 800 (Apple's name for the 9-pin "S800 bilingual" version of the IEEE1394b standard) was introduced commercially by Apple in 2003, allows an increase to 786.432 Mbit/s with backwards compatibility to the slower rates and 6-pin connectors of FireWire 400.

The full IEEE 1394b specification supports optical connections up to 100 metres in length and data rates all the way to 3.2 Gbit/s. Standard category-5 unshielded twisted pair supports 100 metres at S100, and the new p1394c technology goes all the way to S800. The original 1394 and 1394a standards used data/strobe (D/S) encoding (called legacy mode) on the signal wires, while 1394b adds a data encoding scheme called 8B10B (also referred to as beta mode). With this new technology, FireWire, which was arguably already slightly faster, is now substantially faster than USB 2.0.

Almost all modern digital camcorders have included this connection since 1995. All Macintosh computers currently produced have built-in FireWire ports, as do all Sony PCs and many PCs intended for home or professional audio/video use. FireWire is also used on the iPod music player, permitting new tracks to be uploaded in a few seconds and also for the battery to be recharged concurrently with one cable.

Hot swapping warning
Although firewire can technically be hot-swapped without powering down equipment, there are reports of cameras being damaged if the pins are accidentally shorted while swapping. Also, the potential difference between a computer and a camera (see Electrical potential) sometimes results in arcing (sparks) when plugging the camera in. This can in turn "fry" the sensitive firewire chip set of the camera, rendering the firewire port unusable.

To ensure maximum protection of a consumer DV camera, both the camera and computer should be powered off before connecting a firewire cable. Most commercial grade equipment is less sensitive to being hot-plugged, although care should still be taken.

Articles

 * USB 2.0 vs Firewire, benchmarks from PowerPC systems at Bare Feats (2004-05-08)
 * FireWire Port Failures in Host Computers and Peripheral Devices by James Wiebe (archived 2003-08-12)