FireWire is a high speed and low-cost serial bus, initially developed by Apple and Texas Instruments. It allows the connection of up to 63 external devices with a maximum cable distance between them of 4.5 meters, whereas the number decreases to 16 allowing a maximum cable distance of 72 meters. In the ‘80s, Apple Computer Inc. decided to design a new kind of interface to replace the expensive, but at that time very fast, SCSI parallel interface.

It was called FireWire (still an Apple trademark) and soon it focused the interests and the attention of other manufacturers. In particular, Sony started using it under license in its digital video camcorders in 1995, and rapidly extended it on all its digital video and VCRs equipments. The Sony version of the FireWire is also called as iLink or i.LINK (trademarks of Sony). FireWire demonstrated to be a valid type of high-speed interface not only for digital video and audio transfers, but also for any type of digital data.

At that point it was clear that a standard was necessary, so in 1995 the Institute of Electrical and Electronics Engineers introduced the IEEE-1394 specifications. In the following picture is depicted the logo used to identify the FireWire interface.

Nowadays FireWire is being used for digital video camcorders and VCRs, workstations for video-editing, hard disks, and DVD burners/writers. The FireWire original specifications are also known as IEEE-1394a or FireWire 400, since a newer and faster version of FireWire (aka IEEE-1394b or FireWire 800) was introduced in April, 2002. The original FireWire version supported a speed of 100, 200, or 400 Mbps, whereas the latest version is twice as fast as the original one: it can handle up to 800 Mbps. FireWire 800 is backward compatible with FireWire 400, but specific cable adapters have to be used since the connectors are different. The below figure shows a commercial FireWire 400 cable with both the 4 pins and 6 pins connectors. The 4 pins version eliminates the two wire reserved for the DC power supply.

The reader could notice that there is a strong similarity between FireWire and USB, and we could try to identify which of them offers the best performance related to a specific application. USB 2.0, at the moment the newest version of USB available on the market (the specifications of USB 3.0 has just been released at the end of 2008, but we will have to wait some time to see a commercial device featuring this type of interface), was released in 2000 and can achieve a theoretical maximum speed of 480 Mbps. USB 2.0 is also called “Hi-Speed” because of its consistent increase of transfer rate compared to the 1.0 and 1.1 USB versions which offered a maximum speed of 12 Mbps.

A comparison between USB and FireWire shall be conducted identifying three major field of application:

• General purpose peripherals: this field of application is still a prerogative of USB interface. In general we could say that wherever a high transfer ratio is not requested, USB interface is more widely present than FireWire. Examples of such peripherals are: mice, keyboards, scanners, printers, game controllers and peripherals.
• Camcorders and digital video: in this field FireWire is more present and highly competitive compared to USB. From a performance point of view, FireWire 800 offers a higher transfer rate than USB, is directly supported by Apple and present on all Macintosh computers. However, USB is still preferred on most digital cameras because it is common and available on all computers (laptop and desktops). Moreover, FireWire 800 connector is present only on recent digital equipments.
• External high speed peripherals (hard disks, DVD burners and writers): in this sector FireWire is slightly better performing than USB. That’s because it can propose a higher transfer rate (at least in the FireWire 800 version) and because the FireWire bus is able to provide more power than USB, avoiding the use of an external power unit.
  Both FireWire and USB have the common characteristics to be hot-swappable. Nevertheless, FireWire has an important feature that cannot be found in the USB: it does not require a host controller (typically a personal computer) to work, thus allowing two peripheral equipped with that kind of bus interface to exchange data among them (for instance to transfer a digital video from a camcorder to an external hard disk).

The FireWire bus supports two kinds of data transfers: asynchronous and isochronous. The first one involves the usage of handshaking between peripherals and is used mainly for bulk data transfers, as for external hard disks. The latter is instead used to transfer data continuously, in real time, and therefore is more suitable for digital video and audio transfers and acquisition.
All data sent on IEEE-1394 bus is grouped in four bytes words, called “quadlets”. The quadlets are encoded together with their clock signal adopting the Data-Strobe (aka DS) technique, which produces two NRZ (non return to zero) signals. The main advantage of this solution is to ensure that only one of the signals changes in each data bit period.

The above figure shows the behavior of the DS encoding technique. DATA and CLOCK signals are combined together to generate the STROBE signal; precisely, an exclusive OR is applied to them at the transmitter side and only DATA and STROBE are sent on the bus. As shown in the figure, there will only be one change in each data bit period. At the receiver side, conversely, DATA and STROBE signals are exclusive-ored together to reconstruct the CLOCK signal, still accurately locked to the DATA signal. This solution offers the benefit to avoid the need for the receiver to have a PLL (phase locked loop).

FireWire DATA and STROBE signals are sent on the bus in two separate and shielded twisted-wire pairs, aka TPA and TPB. Data transfers which occurs in one direction have the STROBE on TPA and the CLOCK on TPB, whereas the transfers in the opposite direction have the STROBE on TPB and the clock on TPA. The FireWire 6 pin connectors, besides TPA and TPB, includes also two wires for the power supply (power + and power ground). DC power supply ranges from 8 to 33V.