Ask the Wizard

Ask the Wizard answers those questions you always wondered about but never had the courage to ask. Fear not, the Wizard knows all! If you do not find your question answered below, please submit your question. The Wizard


Just what is IEEE 1394?
IEEE 1394 is the official name of the high-performance serial bus technology that Skipstone specializes in. IEEE is the abbreviation for the Institute of Electrical and Electronic Engineers, the standards body controlling the specification for this technology. 1394 is the sequential number of the specification.
I see occasional references to P1394 - what is this?
The P was used in the past to denote the standard when it was provisional. On December 12, 1995, the standard was fully approved and so the P designation should not now be used. Work continues to extend the current specification with these extensions known as 1394.1, 1394.2, etc. Several of these extensions are discussed in the technology questions below.
The name, IEEE 1394, hardly rolls off the tongue. Couldn't you find a better name?
Wizard agrees that a more conversational name might have value and in fact, the 1394 Trade Association is working on finding a name. Stay tuned.
I keep hearing the names FireWire and IEEE 1394 - just what is the difference?
FireWire is a trademarked name of Apple Computer for the IEEE 1394 technology. In practice, the two names represent exactly the same technology. Skipstone can not use the name, FireWire, without Apple's expressed permission.


Do you really think IEEE 1394 will make it?
Wizard thinks this is an impetuous query and recommends doubters check out the who's-who of companies backing 1394 technology. In addition,
  • the Digital Video Consortium has accepted IEEE 1394 as its digital transport standard.
  • Apple Computer has announced support for IEEE 1394 in all Macintosh products by the end of 1996
  • Microsoft has announced that is will support IEEE 1394 in a future release of Windows.

Wizard know all - 1394 technology is the transport of choice for multimedia data transport!

But what about USB?
The Universal Serial Bus (USB) is currently a cost effective solution for connecting low speed devices such as a mouse and a keyboard to a computer. With a maximum data transport rate of 12 Mb/sec, it is less suitable for multimedia data such as digital video and audio. Wizard expects both USB and IEEE 1394 to coexist in future computers.

IEEE 1394

Why is there power in the cable?
There are two uses for this power:
  1. Since this is a serial bus with data being routed from device to device, having one device powered down or malfunctioned would break this data path. The power in the cable maintains each device's connector circuitry thus insuring signal continuity regardless of the state of the device.
  2. For low powered devices, the power in the cable may provide all its power requirements thus avoiding the necessity of an external power connection. Skipstone often uses Sony's prototype desktop camera for its demos where the 1394 cable provides both data and power to this device.
Could you possibly explain this asynchronous and isochronous stuff?
Wizard seems to be feeling somewhat hazy right now but will do his best. Async and Isoch (I do hope we can talk on casual terms here) refers to the two modes of data transfer available with 1394 technology. Most devices will use both modes but this is not required.

Async mode is similar to many of the existing data transport protocols used today. In its simplest form, data is transmitted and the sender waits for an acknowledgment from the receiver that the data was received properly or not. If the data was received properly, the next data is sent and the process continues. If the data was received improperly or this acknowledgment was never returned, an error has occurred and the sender must either resend the original data or initiate some error recovery process. In actual practice, the sender transmit multiple data and uses a queuing process to keep track of corresponding acknowledgments. Async mode therefore transports data whenever the acknowledgment process is satisfied. Since this timing can not be predicted, the data transport timing is somewhat random; thus its name, asynchronous - non-uniform.

Isoch mode implies uniform in time. A sending device knows it needs to transport a specific number of bits per second (i.e., bandwidth). The sending device first negotiates with the other devices on the bus for this bandwidth. Once granted, that bandwidth belongs to the sending device and no other device can contend for it. The sending device now transmits a packet of data at uniform time intervals filling up its bandwidth allotment. This process is extremely efficient as no bus time is wasted waiting for acknowledgments or contesting requests from other devices for bus time. The receipt of Isoch data is thus guaranteed allowing the receiving device to use the data immediately with minimum buffering - just-in-time data. Isoch mode is really what makes 1394 technology so great!

Async mode is often used for command and control signals (e.g., turn the camera on, go to maximum zoom) as well as communicating with legacy devices that already use this data transport mode. Isoch mode is often used to transport the actual data especially when there is a lot of data to be transported.

Well, what if a sending device takes all the bandwidth of the bus for its Isoch transport, won't that block its Async transport and all other devices on the bus?
A device is never allowed to request 100% of the bus bandwidth. For Isoch transport, a single device may request up to approximately 65% of the maximum bus bandwidth and all devices on a bus may not take more that approximately 85% of the bus bandwidth. The remaining bandwidth insures that some Async data will always be able to get through.

Wizard is getting tired from these techie questions and thinks you should read the 1394 specification. The specification is IEEE 1394-1995, Product Number SH94364 and it is available from:

IEEE Computer Society
Washington, DC, USA
Phone: 1-800-678-IEEE, or 1-908-562-3829
For my application, the 4 meter cable length needs to be longer. Any suggestions?
The current specification limits cable length to 4 meters. There are two solutions to longer cable length, although neither are currently available:
  1. Manufacturers will offer repeater boxes that will extend the cable length an additional 4 meters. Multiple repeaters can be used for increased distances. Contact us if you have such a need.
  2. The 1394 Trade Association is working on the specification for a new cable that allows a much longer cable length.
Speaking of new stuff, what is this gigabit speed I keep hearing about?
Wizard wonders who you have been talking to. Well it is true. The 1394 Trade Association is also working on extending the current data transfer rate maximum of 400 megabit per second to over 1 gigabit per second. Gee, now everyone knows.

Digital Video

I just bought a Sony VX-1000 Digital Video Camcorder and ...
There is currently no way to transfer digital data from IEEE 1394 cameras, such as the Sony VM-1000 Camcorder, to a computer. We are currently working to solve this problem.
Well, I really am anxious to use Sony's digital I/O capability!
Wizard recommends you try exercise and meditation to reduce anxieties in one's life. In the meantime, contact us and we will keep you informed about new products.
I noticed that the Sony VX-1000 Camcorder has a different IEEE 1394 cable connector than shown in the IEEE 1394 literature - what gives?
There are now two cables. The original IEEE 1394 cable had 6 wires. Sony's cable has 4 wires since the 2 power wires were removed. Wizard expects to see cable converters in the future.

1394 Trade

Why should I join the 1394 Trade Association?
Wizard likes to travel to exotic lands every three months to wine and dine with his friends. There is also a good deal of work going on in the association's subcommittees to further refine and extend the specification as well as to insure that the data transported over IEEE 1394 is compatible among different companies.
OK, how do I join?
Wizard thought you would never ask - just check out the TA's Web page.

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