IEEE 1394
Eugen Leitl
eugen at leitl.org
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):
http://developer.apple.com/firewire/IP_over_FireWire.html
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."
Latency:
http://www.evaluationengineering.com/archive/articles/0801pcbased2.gif
Throughput:
http://www.evaluationengineering.com/archive/articles/0801pcbased1.gif
The article is at
http://www.evaluationengineering.com/archive/articles/0801pcbased.htm
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 you.re 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
watts.
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
MB/s.
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
superior.
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, we.ve 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
devices..
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.
Reference
1. Graham, J., .Approval Expected for USB On-The-Go,. Electronic
Buyers. News, www.ebnews.com, March 8, 2001.
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