Firewire Bus (IEEE 1394)


With the new FireWire revisions working to make it a serial bus with better performance each day, the communication rate implemented by a bridge is flexibly programmable to be between S100 (100 Mb/s) and S3200 (3200 Mb/s), with accessible cost either to connect computer peripherals or appliances. According to the 1394 committee, other devices, such as digital video transmission, are still limited by an architecture and a definition of protocol to bridges that are incomplete nowadays. In fact, the bus physical specification was very simple. The most complex is to define the bridge patterns, what is being done by IEEE 1394-1 proposition.

To start a description of 1394-1 proposition, some definitions will be shown.

  • Bridge: it is the circuit capable of allowing the communication between two or more serial buses with independent operations.

  • Bus_ID: it is a number of 10 bits that identifies, in a single way, one of the serial buses in a network topology made up of many buses.

  • Portal: it is the circuit that physically connects a FireWire bridge to a bus. Each bridge should implement at least 2 portals, that is, allow the communication between 2 buses. In a generic network, a bridge can implement various portals, identified by 0, 1, …, N-1.

  • Network: it is a group of interconnected buses and nodes, capable of being mutually addressed by transactions involving data packets.

  • Local Node: Two nodes are said to be local if they are connected to the same bus, that is, with the same Bus_ID.

  • Local Node ID: it is a number of 16 bits that will represent an address to the node (peripheral) connected to the bus.

  • Network Cycle Master: it is a circuit elected to be the responsible for giving the clock that will synchronize the network. This way, many network events could happen synchronized by a single clock (isochronal way). But, the pattern IEEE 1394-1 also mentions an asynchronous communication.

  • Physical ID: it is a number of 6 bits associated to each node, through auto-identification process that comes after the bus initialization.

  • Remote Node: A node said to be remote in relation to another if they are connected to buses with different IDs.

  • Virtual Node: A generic node.

  • Virtual ID: it is a number of 6 bits that represents an address of a connected local node. The association of those Ids is made through the portal that generated the local bus.

  • Virtual Node ID: it is a number of 16 bits that represents an address to a generic node that, to transaction packets with a determined portal, should go over at least one bridge.

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Author: Ricardo Zelenovsky

Ricardo Zelenovsky, PhD, is professor at UNB (Universidade de Brasília), a Brazilian Engineering University. Together with Alexandre Mendonça, Zelenovsky wrote several books about computer hardware. Visit their website at

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