FireWire (IEEE 1394) [IEEE 1394]


(n.) An interface standard that specifies a transmission method, medium, and protocol. This high-performance serial bus defines a point-to-point cable-connected virtual bus and a backplane physical layer. The cable version supports data rates of 100, 200, 400 and 800 megabits per second (Mbps) over a cable medium supported by the standard. The backplane version operates at 12.5, 25, or 50 Mbps. Both versions are fully compatible at the link layer and above. Features include multimaster capabilities, live connection and disconnection (hot plugging), and transmission speed scalable from 100 to 800 Mbps.

The technology was developed by Texas Instruments to address the need for mass information transfer. Ordinary networks do not provide connection capabilities and bandwidth that can meet future demands. Parallel high-speed communications, such as SCSI, are not suited to long-distance transmission and do not support live connection and disconnection of peripherals, such as digital video cameras, scanners, or printers. This capability is provided by the 1394 standard. The cable version integrates I/O connectivity in personal computers using a scalable, high-speed serial interface.

The standard is a transaction-based packet technology for both chassis and peripheral devices. The serial bus behaves as though it were memory space interconnected between devices or as if devices resided in slots on the main backplane. Device addressing is 64 bits wide, allocating 10 bits for network IDs, 6 bits for node IDs, and 48 bits for memory addresses. It is capable of addressing 1023 networks with 63 nodes on each. The memory capacity for each device on the chain is 256 terabytes (TB).

Memory-based addressing views resources as registers or as memory that can be accessed with processor-to-memory transactions. A bus entity is referred to as a node, which may be individually identified, addressed, and reset. Multiple nodes may be in a single module, and multiple ports may be in a single node.

The distance between each node should not exceed 4.5 meters, and the maximum number in a chain is 16, for a total maximum end-to-end distance of 72 meters. Cable distance between each node is limited primarily by signal attenuation. A cable with 28-gauge signal pairs may be up to 4.5 meters long, whereas a cable with 24-gauge signal pairs may be 14 meters long. A maximum of 16 cable hops is allowed between the most widely separated nodes. The end-to-end distance may vary from 72 to 224 meters, depending on the configuration.

Signals transmitted on cable and backplane environments are non-return-to-zero (NRZ) with Data-Strobe (DS) encoding. DS encoding allows only one of the two signal lines to change each data bit period, doubling the tolerance jitter. DS encoding is licensed from SGS-Thompson/INMOS.

The FireWire IEEE 1394 standard supports asynchronous and isochronous data transfers. The asynchronous format transfers data and transaction layer information to a specific address, whereas the isochronous format broadcasts data based on channel numbers rather than on specific addressing. Both nonrealtime critical applications (such as scanners and printers) and realtime critical applications (such as video and audio) can operate on the same bus because the interface provides both formats.

The serial bus 1394 replaced other peripheral connection communication methods formerly in use, such as the Centronics parallel, RS-232, SCSI, and Apple Desktop Bus (ADB). Newer interfaces, such as direct connect video I/O, used this technology because of its advantages. Memory space addressing is a good solution for “slotless” systems. Communications at different speeds between 100 and 800 Mbps may occur simultaneously on one medium. The “hot-plugging” and dynamic reconfiguration abilities make it a user-friendly environment. This standard will allow a user to connect an expansion system with communications on demand without having to shut down and reconfigure whenever devices are added or removed.

In June of 2007, the 1394c specification was published, which provided data transfer at 800 Mbit/s over Category 5 cable. Other than Sony digital cameras, few devices were developed that employed the Firewire interface after 2008.