IEEE 1394

Eugen Leitl eugen at
Thu Dec 5 02:47:59 PST 2002

After I've posted a link to the Oracle library for clustering over 1394 
a while back Apple mumbled something about RFC 2734 (IP over 1394):

 All Apple computers sold today include one or more FireWire ports. 
 Because FireWire can transfer data at up to 400 megabits/second, it is 
 suitable for networking and clustering solutions, as well as temporary 
 connections to the internet using Internet Sharing.

 Now the IP over FireWire Preview Release adds support for using the 
 Internet Protocol - commonly known as TCP/IP - over FireWire. With this 
 software installed, Macintosh computers and other devices can use existing 
 IP protocols and services over FireWire, including AFP, HTTP, FTP, SSH, 
 etc. In all cases, Rendezvous can be used if desired for configuration, 
 name resolution, and discovery.

 The preview release adds a new Kernel Extension that hooks into the 
 existing network services architecture. Using the existing Network 
 Preferences Pane, users can add FireWire as their IP network node to 
 connect and communicate between two machines.

 Now developers interested in using the Internet Protocol (IP) over 
 FireWire may download the IP over FireWire Preview Release.

[the document is in some proprietary .dmg format]

Since I've never seen real numbers for IEEE 1394 latency I did some 
websearches, and finally found some meat:

"The IEEE 1394 bus has a minimum latency of a few hundred microseconds and 
a worst-case delay of a few milliseconds. For large data blocks, this bus 
uses direct memory access (DMA) similar to PCI bus mastering that reduces 
the influence of software protocol overhead on the transfer rate. The 
400-Mb/s top data rate supports consumer digital video equipment and data 
acquisition devices requiring relatively fast data transfer. Bus latencies 
are compared in   Figure 1 and bus throughput in   Figure 2."


The article is at

Relax and Take the Bus

by Tom Lecklider, Technical Editor

Depending on your experience, the phrase PC-based instrumentation conjures 
up a variety of images. You may be contemplating using PC-based test for 
the first time, you may have used it in the past, or you may think it.s 
not suitable for your present application. If in the last group, 
make sure you know how both PCs and instruments have changed before you 
make a final decision.

On the PC side, two new buses have addressed the time when PCs will have 
no internal expansion slots. And, instruments have become smarter and 
leaner. Recently developed low-power semiconductor technology has provided 
high-speed digital signal processors (DSPs) and high-precision 
analog-to-digital converters (ADCs) that can operate from only a few 

Instrument design has made better trade-offs between essential hardware 
functions and those things that can be done well by a Pentium-class host. 
The end result is smaller, lower cost instrumentation in a convenient and 
easy-to-use form.

When they were introduced, the universal serial bus (USB) and the IEEE 
1394 bus (aka FireWire from its Apple Computer roots) were easily 
distinguished by their relative speeds. USB version 1.1 (USB1) peripherals 
include mice, keyboards, floppy disk drives, and other devices with bit 
rates below 12 Mb/s.

In fact, the USB1 specification established both a low-speed 1.5-Mb/s mode 
and a high-speed 12-Mb/s mode. However, neither was very impressive 
compared to FireWire.s 400-Mb/s rate.

On the other hand, whether due to better PR, FireWire license fees, or the 
very low cost of USB peripherals, USB has clearly eclipsed FireWire at 
this time. Virtually all PCs ship with built-in USB ports, but only a few 
computer manufacturers include FireWire.

So, one major USB advantage is ubiquity. Another is the recent version 2.0 
(USB2) specification, calling for a top rate of 480 Mb/s. Of course, 
nothing comes for free, and comparable speed doesn.t necessarily equate to 
equivalent performance.

The Buses Compared

Both USB and FireWire are serial buses with hot-plug-and-play 
capabilities. This feature allows you to safely add or remove devices from 
the bus while the PC and any connected bus hubs are powered. There is one 
major difference between the buses: USB always requires a PC master while 
the 1394 bus provides peer-to-peer communications without PC intervention.

USB1 transfers a 1,500-B frame every millisecond, and the frame is shared 
by all connected USB devices.up to a maximum of 127. This means that 
actual data transfer for any one device could be as slow as one data point 
per two or three frames, although the useful composite rate is about 1.16 
Passing all communications through the central PC makes possible very 
low-cost USB peripherals because they require minimal intelligence. The 
downside is increased transfer latency. It.s quite variable but as high as 
8 or 9 ms for USB1. For that reason, USB1 is not a good bus choice for 
single-point transfers because its high latency limits you to low-speed 
monitoring and slowly changing temperature or pressure measurements.

USB2.s 480-Mb/s top rate improves burst transfer speed greatly. Its 
latency also improves because of the new 125-µs microframes rather than 
USB1.s 1-ms frames.

But, as described by Andy Purcell, a software design engineer at Agilent 
Technologies, .USB2 still is a master-slave architecture and will have an 
inherent fixed latency. The latency occurs because a USB slave cannot just 
send data when it is available. It must wait to send the data until asked 
for it. The latency is independent of CPU speed..

The IEEE 1394 bus has a minimum latency of a few hundred microseconds and 
a worst-case delay of a few milliseconds. For large data blocks, this bus 
uses direct memory access (DMA) similar to PCI bus mastering that reduces 
the influence of software protocol overhead on the transfer rate. The 
400-Mb/s top data rate supports consumer digital video equipment and data 
acquisition devices requiring relatively fast data transfer. Bus latencies 
are compared in Figure 1 and bus throughput in Figure 2.

Just as the USB specification has been upgraded, so too is there a 1394b 
version that will supersede the present 1394a. The proposed changes extend 
the top 400-Mb/s rate to 800, 1,600, and ultimately 3,200 Mb/s. However, 
although USB2 retains common protocol and operation with USB1, 1394b may 
not be entirely backward compatible with 1394a. Until the dust settles, 
manufacturers haven.t committed to 1394b silicon, preferring to back an 
unambiguous USB2.

To extend the USB realm of addressable applications further, the USB 
Implementers Forum (USB-IF) has proposed a USB On-The-Go subset of USB2. 
This specification enhancement would allow USB peripherals to exchange 
information directly, without the need for an intervening PC. So, you 
could download images from your digital camera directly to your printer 
without having to go through a PC between the two devices. On the other 
hand, a PC would be required if, for example, you wanted to crop, enhance, 
or otherwise edit the image prior to printing or if you needed to archive 
it on disk.

According to a recent article by Jeanne Graham, .USB is not necessarily a 
better technology than Bluetooth or 1394, but it has deployed better 
marketing campaigns.. Ms. Graham also quoted Bert McComas, an analyst at 
InQuest Market Research: .A consumer product manufacturer will say, .Give 
me one good reason to go with USB.. Well, one good reason is that every PC 
in the world has a USB port..1

The Industrial Case

The USB.s advantages for consumer applications seem to be equally valid 
for industrial users. Ease of use, low cost, and worldwide independence 
from AC supply considerations influenced Herb Figel.s decision to purchase 
a Dactron Photon Spectrum Analyzer.

Mr. Figel, the director of quality assurance at Hunter Fan, already had 
some experience with USB peripherals, having previously bought a 
digitizing pad and a device to synchronize his Palm organizer with his PC. 
He commented that the USB spectrum analyzer provided similar measurements 
to an older, large bench instrument, but that its user interface was much 

The Photon instrument has an upper frequency limit of 21 kHz and is 
entirely powered by the USB connection. It was a good fit to Mr. Figel.s 
ceiling-fan noise-measurement application with frequencies in the 100-Hz 
to 1-kHz range. Had he needed multimegahertz speeds, he wouldn.t have 
found an instrument that operated within the USB.s meager 2.5-W power 
limit, although fast PCI-bus cards are readily available. So, he could 
have retained a PC-based test system, but it wouldn.t have been as simple 
and convenient as that made possible by USB.

As an example, the Gage Applied CompuScope 14100 is a dual-channel, 
100-MS/s, 14-bit resolution PCI card. It achieves sustained 100-MB/s data 
transfer rates via PCI bus mastering under single-tasking operating 
systems. On-board memory ranges from 1 MS to 1 GS, and the card draws from 
25 to 35 W.

For Gage.s customers, a high sustained data-transfer rate is important. 
.High bus transfer speed, while almost irrelevant in one-shot applications 
like explosion testing, is essential in the acquisition of repetitive 
signals,. explained Andrew Dawson, the company.s product manager of 
board-level products and advanced measurement systems. .Examples of these 
applications include radar, lidar, ultrasonic imaging, and manufacturing 
test systems. A typical requirement is to capture 1,000 point acquisitions 
at a repetition rate of over 10 kHz without missing a single event..

Also shunning USB and FireWire for the moment is Mark Cejer, the test and 
measurement marketing manager at Keithley Instruments. .Until an 
instrument comes along that offers unique features only available with USB 
or FireWire, there probably will be little incentive for users to buy 
them. Large production ATE racks consist of multiple types of instruments. 
What good will it do to have a USB DMM, for example, if all the other 
instruments are GPIB?.

Balancing this view is one that considers the need to connect new PCs to 
existing GPIB and RS-232 instruments. National Instruments. solution 
consists of the GPIB-USB-A and the GPIB-1394 controllers that transform 
any computer with a USB or FireWire port into a plug-and-play GPIB 
controller that can handle up to 14 instruments.

For Dewetron, system simplification is a theme that runs parallel with the 
development of the DEWE-BOOK. Grant Smith, the company president, said, .A 
DEWE-BOOK is an eight- or 16-channel signal-conditioning front end with a 
built-in ADC that precedes our DAQ and PAD series of modules. Previously, 
we offered an internal ADC board that had to be connected to the PC.s 
printer port, but as well as tying up the printer port, it limited 
throughput to 20 kHz. Today, we get a very consistent 100-kHz throughput 
with each USB-connected DEWE-BOOK.

.In addition, it is plug-and-play, making it easier for our customers to 
install and get running, and USB is well supported by available software. 
Also,. he continued, .we had to use a separate COM port to control the 
settings on our DAQ and PAD modules. This meant that both a parallel port 
and a COM port on the customer.s PC were tied up. Now, we handle all 
control as well via USB. As a final benefit, by adding a hub, up to four 
DEWE-BOOKs can be used simultaneously with the same PC..

The Broader Picture

Of course, USB and FireWire are just two of many instrumentation and 
computer buses available today. Agilent.s Mr. Purcell said, .USB1 block 
transfer performance is similar to GPIB, so the primary benefit for USB is 
the ease with which customers can connect instruments to PCs. FireWire 
performance is quite good, and we are seeing block transfer rates of 15 
MB/s. This is 20 times better than GPIB and more than 1,000 times better 
than RS-232.

.USB2, with its Intel backing, may become as pervasive as USB1 is today. 
Its higher bit rate should enable 10 times the performance of GPIB,. he 
continued. .In anticipation of this, there is a growing international 
group of companies that has started work on a standard USB protocol for 
test and measurement devices..

As a step toward industry-wide software compatibility, the VXIplug&play 
Systems Alliance has developed a specification for I/O software called 
Virtual Instrument Software Architecture (VISA). VISA provides a common 
foundation for the development, delivery, and interoperability of 
high-level multivendor system software components, such as instrument 
drivers, soft front panels, and application software. VISA not only allows 
test engineers to combine different I/O buses into one system, but also 
provides the necessary abstraction layer to make the transition to new 
buses transparent to the user.

.Although VISA solves the mixed I/O problem on the host side,. commented 
Vanessa Trujillo, an instrument connectivity product manager at National 
Instruments, .a similar architecture is needed on the device side to make 
the integration of new bus types seamless for the instrument manufacturer. 
For instrument manufacturers to embrace and adopt new buses while at the 
same time to support their many customers who still use one of today.s 
buses, they need an architecture that allows them to easily adapt the 
firmware they have written for one bus type to another..

Sharing Mr. Purcell.s USB emphasis, Nick Turner, sales and marketing 
manager at Cytec, said .The advantage we saw with USB was the ability to 
daisy-chain devices to run from one PC port. RS-232 is a one-to-one bus, 
and GPIB is limited to 16 devices, so we thought there probably would be 
interest in USB. However, not done anything with FireWire because it 
requires licensing..

Mr. Turner cited the USB.s 5-m length restriction as a disadvantage in his 
company.s automated test business. Although greater expense and complexity 
accrue, hubs can be stacked to a maximum of 30 m. The 5-m length limit 
also applies to each FireWire hop, but the 1394b version promises to span 
100-m or greater distances by matching media and speed. For example, 100 m 
could be achieved at high speed via fiber-optic cable, where copper would 
be appropriate at lower speeds.

The continuing adoption of Ethernet for instrument communications goes on 
in the background as USB and FireWire vie for position. Mr. Turner 
commented, .The biggest trend we currently are seeing is more and more 
people wanting devices with a network interface. There is a plethora of 
software and hardware support for 100Base-Tx and even Gigabit Ethernet, 
and people are becoming increasingly accustomed to working with them..

Agilent.s Mr. Purcell agreed, but added a cautionary note. .Ethernet 
connections allow instruments to communicate using http, RMI, DCOM, and 
RPC. Instruments can act as web servers, and users can use familiar 
browsers to control and view collected data. There seems to be a lot of 
enthusiasm for connecting instruments to Ethernet, but enhancements are 
necessary so that it.s easy to configure and then discover attached 

Going beyond bus-tethered instruments, Gage Applied.s Dr. Dawson foresees 
high-speed wireless links that will transform PC-based test and 
measurement. .Within a few years, the primary human interface to the PC 
will not be the traditional mouse, keyboard, and monitor. Instead, users 
will interface to a portable personal digital assistant (PDA) that, in 
turn, will communicate through a wireless interface to a faceless 
connected instrument (FCI). Communications via the PDA will allow greater 
data sharing and free the user to control equipment remotely in a manner 
unavailable today,. he explained.


   1. Graham, J., .Approval Expected for USB On-The-Go,. Electronic 
Buyers. News,, March 8, 2001.

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