FireWire Bus (iLink / IEEE 1394)

Presentation of FireWire Bus (IEEE 1394)

The IEEE 1394 bus (name of the standard to which it makes reference) was developed at the end of 1995 in order to provide an interconnection system that allows data to circulate at a high speed and in real time. The company Apple gave it the commercial name "FireWire", which is how it is most commonly known. Sony also gave it commercial name, i.Link. Texas Instruments preferred to call it Lynx.

FireWire is a port that exists on some computers that allows you to connect peripherals (particularly digital cameras) at a very high bandwidth. There are expansion boards (generally in PCI or PC Card / PCMCIA format) that allow you to equip a computer with FireWire connectors. FireWire connectors and cables can be easily spotted thanks to their shape as well as the following logo:

FireWire Logo

FireWire Standards

There are different FireWire standards that allow you to obtain the following bandwidths:

Standard Theoretical Bandwidth
IEEE 1394a
IEEE 1394a-S100 100 Mbit/s
IEEE 1394a-S200 200 Mbit/s
IEEE 1394a-S400 400 Mbit/s
IEEE 1394b
IEEE 1394b-S800 800 Mbit/s
IEEE 1394b-S1200 1,200 Mbit/s
IEEE 1394b-S1600 1,600 Mbit/s
IEEE 1394b-S3200 3,200 Mbit/s

The IEEE 1394b standard is also called FireWire 2 or FireWire Gigabit.

FireWire Connectors

There are different FireWire connectors for each of the IEEE 1394 standards.

  • The IEEE 1394a standard specifies two connectors:
    • Connectors 1394a-1995:

      connector 1394a-1995

    • Connectors 1394a-2000, called mini-DV because they are used on Digital Video (DV) cameras:

      connector 1394a-2000

  • The IEEE 1394b standard specifies two types of connectors that are designed so that 1394b-Beta cables can be plugged into Beta and Bilingual connectors, but 1394b Bilingual cables can only be plugged into Bilingual connectors:
    • 1394b Beta connectors:

      1394b Beta connector

    • 1394b Bilingual connectors:

      1394b Bilingual connector

How the FireWire Bus Works

The IEEE 1394 bus has about the same structure as the USB bus except that it is a cable made up of six wires (2 pairs for the data and the clock and 2 wires for the power supply) that allow it to reach a bandwidth of 800 Mb/s (soon it should be able to reach 1.6 Gb/s, or even 3.2 Gb/s down the road). The two wires for the clock is the major difference between the USB bus and the IEEE 1394 bus, i.e. the possibility to operate in two transfer modes:

  • Asynchronous transfer mode: this mode is based on a transmission of packets at variable time intervals. This means that the host sends a data packet and waits to receive a receipt notification from the peripheral. If the host receives a receipt notification, it sends the next data packet. Otherwise, the first packet is resent after a certain period of time.
  • Synchronous mode: this mode allows data packets of specific sizes to be sent in regular intervals. A node called Cycle Master is in charge of sending a synchronization packet (called a Cycle Start packet) every 125 microseconds. This way, no receipt notification is necessary, which guarantees a set bandwidth. Moreover, given that no receipt notification is necessary, the method of addressing a peripheral is simplified and the saved bandwidth allows you to gain throughput.

Another innovation of the IEEE 1394 standard: bridges (systems that allow you to link buses to other buses) can be used. Peripheral addresses are set with a node (i.e. peripheral) identifier encoded on 16 bits. This identifier is divided into two fields: a 10-bit field that identifies the bridge and a 6-bit field that specifies the node. Therefore, it is possible to connect 1,023 bridges (or 210 -1) on which there can be 63 nodes (or 26 -1), which means it is possible to address 65,535 peripherals! The IEEE 1394 standard allows hot swapping. While the USB bus is intended for peripherals that do not require a lot of resources (e.g. a mouse or a keyboard), the IEEE 1394 bandwidth is larger and is intended to be used for new, unknown multimedia (video acquisition, etc.).

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