FireWire bus (IEEE 1394-1995)

FireWire (also known as i.Link or IEEE 1394) is a personal computer and digital video serial bus interface standard offering high-speed communications and isochronous real-time data services. The IEEE 1394-1995 standard for the High Performance Serial Bus defines a serial data transfer protocol. IEEE 1394 is going to be the interface for connecting handy-cams and VCRs, settop boxes and televisions. The capabilities of the 1394 bus are sufficient to support a variety of high-end digital audio/video applications, such as consumer audio/video device control and signal routing, home networking, nonlinear DV editing, and 32-channel (or more) digital audio mixing.

FireWire 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.

• Real-time data transfer for multimedia applications
• 100 - 200 - 400 Mbits/s data rates
• Live connection/disconnection without data loss or interruption
• Automatic configuration supporting "plug and play"
• Freeform network topology allowing mixing branches and daisy-chains
• No separate line terminators required
• Guaranteed bandwidth assignments for real-time applications
• Common connectors for different devices and applications
• Compliant with IEEE-1394 High Performance Serial Bus standard

1394 is based on Apple Computer"s original 1394 bus, which was intended as a low-cost replacement for or supplement to the SCSI bus that is a standard feature of Macintosh and PowerMac computers.

IEEE - 1394 Architecture

The 1394 standard defines two bus categories: backplane and cable. The backplane bus is designed to supplement parallel bus structures by providing an alternate serial communication path between devices plugged into the backplane. The cable bus, which is the subject of this paper, is a "non-cyclic network with finite branches," consisting of bus bridges and nodes (cable devices). Six-bit Node_IDs allow up to 63 nodes to be connected to a single bus bridge; 10 bit Bus_IDs accommodate up to 1,023 bridges in a system. This means, as an example, that the limit is 63 devices connected to a conventional 1394 adapter card in a PC.

Each node usually has three connectors, although the standard provides for 1 to 27 connector per a device"s physical layer or PHY. Up to 16 nodes can be daisy-chained through the connectors with standard cables up to 4.5 m in length for a total standard cable length of 72 m. 1394 truly qualifies as a plug-and-play bus.

The 1394 cable standard defines three signaling rates: 98.304, 196.608, and 393.216 Mbps (megabits per second; MBps in this paper refers to megabytes per second.) These rates are rounded to 100, 200, and 400 Mbps, respectively, in this paper and are referred to in the 1394 standard as S100, S200 and S400. Consumer DV gear uses S100 speeds, but most 1394 PC adapter cards support the S200 rate. The signaling rate for the entire bus ordinarily is governed by the slowest active node; however, if a bus master (controller) implements a Topology_Map and a Speed_Map for specific node pairs, the bus can support multiple signaling speeds between individual pairs.

Cables and Connectors

Standard bus interconnections are made with a 6-conductor cable containing two separately-shielded twisted pair transmission lines for signaling, two power conductors, and an overall shield. The two twisted pairs are crossed in each cable assembly to create a transmit-receive connection. The power conductors (8 to 40 v, 1.5 a max.) supply power to the physical layer in isolated devices. Transformer or capacitative coupling is used to provide galvanic isolation; transformer coupling provides 500 volts and lower-cost capacitative coupling offers 60 volts of ground potential difference isolation. Connectors are derived from the GameBoy design.

IEEE-1394Bus Management

1394 provides a flexible bus management system that provides connectivity between a wide range of devices, which need not include a PC or other bus controller. Bus management involves the following three services:

• A cycle master that broadcasts cycle start packets (required for isochronous operation)
• An isochronous resource manager, if any nodes support isochronous communication (required for DV and DA applications)
• An optional bus master (a PC or an editing DVCR might act as a bus master)

On bus reset, the structure of the bus is determined, node IDs (physical addresses) are assigned to each node, and arbitration for cycle master, isochronous resource manager, and bus master nodes occurs.

Isochronous Data Transport

The isochronous data transport of the 1394 bus provides the guaranteed bandwidth and latency required for high-speed data transfer over multiple channels. On bus reset or when an isochronous node is added to the bus, the node requests a bandwidth allocation. If adequate bandwidth is not available, the requesting device is expected to repeat its request periodically.

Video acquisition for non-linear digital editing is simpler than the camcorder-DVCR example, because it requires only a single isochronous channel, plus an asynchronous path for device control.

EEE 1394 is a standard, platform-independent solution. Its features represent an evolutionary improvement over current I/O interfaces and provide connectivity solutions for many markets. Legacy I/O bridges attach serial and parallel interfaces to 1394. ANSI SCSI-3 provides a migration path for parallel SCSI to move to IEEE 1394. IEEE 1394 can interface with the higher layers of the new parallel port standard, IEEE 1284. Although IEEE 1284"s 4 to 32 Mbps transfer rate is lower than that of 1394, 1284 finds application in printer connectivity since it is backward compatible with the existing Centronics parallel port. IEEE 1394 devices of differing transport rates may be interconnected, allowing backward compatibility with devices having slower transport rates. This feature allows 100 Mbps devices purchased today to operate properly in future bus configurations involving 200 and 400 Mbps devices.