Electronic interfaces/IEEE-1394

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1 IEEE-1394
2 References

IEEE-1394

The IEEE 1394 interface is a serial bus interface standard for high-speed communications and isochronous real-time data transfer, frequently used in a personal computer, digital audio and digital video. The 1394 digital link standard was conceived in 1986 by technologists at Apple Computer, who chose the trademark 'FireWire', in reference to its speeds of operation. The first specification for this link was completed in 1987. It was adopted in 1995 as the IEEE 1394 standard. The interface is also known by the brand names of i.LINK (Sony) and Lynx (Texas Instruments). FireWire can connect up to 63 peripherals in a tree topology and 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. The newer, current IEEE 1394b (Fire standard was introduced commercially by Apple in 2003. It allows a transfer rate of 786.432 Mbit/s full-duplex via 9 pin connector, and is backwards compatible to the 6 pin configuration and slower data rates of the original IEEE 1394a standard. The full IEEE 1394b specification supports data rates up to 3200 Mbit/s over beta-mode or optical connections up to 100 metres in length. In December 2007, the 1394 Trade Association announced that products will be available before the end of 2008 using the S1600 and S3200 modes that, for the most part, had already been defined in 1394b. The 1.6 Gbit/s and 3.2 Gbit/s devices will use the same 9-pin connectors as the existing FireWire 800 and will be fully compatible with existing S400 and S800 devices. It will compete with the forthcoming USB 3.0.^[1]. IEEE 1394b is used in military aircraft and automobiles, where weight savings are important, as well as with computer vision and digital video cameras.

IEEE-1394 has the following features:

Real-time data transfer for multimedia applications
100 and 200Mbits/s data rates today; 400- 800 Mbits/s and mul ti-Gbits/s upgrade path
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

Cables and connectors

Standard bus interconnections are made with a 4,6 or 8 - conductor cable containing two separately-shielded twisted pair transmission lines for signaling, two power conductors, and an overall shield (see figure 3). 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.

			IEEE 1394 connector configurations
4-pin	6-pin	9-pin	Name	Color	Description
	1	8	Power	white	Unregulated DC; 30 V no load
	2	6	Ground	black	Ground return for power and inner cable shield
1	3	1	TPB-	orange	Twisted-pair B, differential signals
2	4	2	TPB+	blue	Twisted-pair B, differential signals
3	5	3	TPA-	red	Twisted-pair A, differential signals
4	6	4	TPA+	green	Twisted-pair A, differential signals
		5	A shield
		7
		9	B shield
Shell	Outer	cable shield

Data

IEEE 1394 specifies two signalling mechanisms across the bus: common mode (DC) and differential. DC signalling means that a logical 1 is represented as a positive voltage, and a logical 0 is represented as zero voltage. In 1394, it is used for three purposes: device attachment and detection, speed signalling, and power management (suspend-resume). Differential signalling means that logic values are represented by the difference between two wires: if the voltage on one wire is greater than the next, then a logical 1 is represented, otherwise a logical 0 is represented. The advantage of this method is that power consumption is greatly reduced. As such, 1394 uses differential signalling for the majority of operations, such as bus reset, arbitration, configuration commands, and data packet transmission. In addition to the regular data packets, there are several "special" PHY (physical) packets to deal with configuration and arbitration information passing. These packets are grouped into four categories: the self-ID packet, the link on packet, the PHY configuration packet and the extended PHY packets.

Self-Identification Packet: The PHY unit sends these packets during the self-ID phase of arbitration, or when other devices on the bus send a "ping" packet. Up to three self-ID packets may be sent in response – the exact number is implementation dependent. Along with other parameters, the self-ID packet(s) contain information about the maximum communication speed supported by this device, the port connection status, and power consumption characteristics.
Link-On Packet: The purpose of the link-on packet is to inform the recipient PHY that it should expect to receive additional packets. As such, the link-on packet consists only of the recipient's PHY number. The packet is broadcast to every node on the bus, and the recipient whose PHY number matches the number specified in the packet is required to set its internal state to LINK_ON.
PHY Configuration Packet: The PHY configuration packet is used to configure various parameters of the bus in order to improve performance. The parameters that are configurable are the "gap count" (i.e. delay between isochronous packets) and the setting of the "root" node of the bus.
Extended PHY Packets: The purpose of the extended PHY packets are for other nodes on the bus to query the status of the target node, by means of sending these packets. The "ping" packet described earlier is an example of an extended PHY packet; the recipient node sends a self-ID packet, with the response time field in this packet filled in. Another useful extended PHY packet is the remote access packet, which lets a remote node read any of the target PHY's status registers -- these are reported back to the initiator PHY by means of another extended PHY packet, the remote reply packet. Remote nodes are also permitted to ask a node to execute one of five operations (commands) by sending a remote command packet. The permitted operations are: disable and enable port, enable and disable suspend mode, and clear faults. Naturally, the recipient node sends a remote confirmation packet to indicate to the initiator when the requested command has completed. The final type of extended PHY packet permitted is a resume packet -- there is no reply packet associated with this command, however. The purpose of the packet as the name suggests is to instruct the recipient to resume from suspend mode.
Data PHY Packets:The data payload comprises a very small portion of the physical layer packet; the bulk of the packet contains arbitration and transaction type information. The various fields aside from the payload are:
- Destination address - consisting of target bus, node, and device within that node
- Source address - consisting of the source bus and node address
- Transaction type - type of packet being sent. There are twelve packet types possible, but a discussion of them is beyond the scope of this document.
- Transaction label - allows matching of requests and acknowledgements (like a sequence number
- Response code - part of a packet sent in response to a request for "completion status".
- CRC - standard error-checking checksum for a request or response packet.
- Acknowledge code - returned by the receive of a packet to verify receipt.
- Acknowledge parity - a simple parity bit for the acknowledge code packet, since full CRC would be too expensive for a simple acknowledge code.

Further data format detail