Leland Teschler
Executive Editor
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Here is something you can't do right now: Connect an amplifier from Aerotech Inc. (left) on the same FireWire network as an amplifier from Ormec Systems Corp. (right) The reason is both suppliers use their own high-level protocols for controlling their equipment over FireWire networks, and the two high-level protocols don't talk to each other. The idea behind the new 1394AP standard is to define a common interface that could let FireWire equipment from different industrial vendors interoperate for real-time applications. Neither Aerotech nor Ormec has yet evaluated 1394AP. |
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The structure of the1394AP protocol becomes evident from this diagram constructed by developers at the Fraunhofer Institute. With 1394AP, one node of the network works as an application master while the rest work as slaves. The master cyclically transmits data to the slaves. Slaves may exchange data with the master and with each other as in the case of neighboring axes in a coordinated control system. The AP part of the protocol defines only a data structure for the transmission of application-specific information. Interpretation of the data takes place in the next higher level of the protocol, which is not spelled out in AP. Fraunhofer thinks a likely candidate for this higher level of the protocol is the CAN standard. |
Examine any camcorder or laptop computer and you will likely find a socket for a FireWire plug. FireWire, also known as IEEE 1394, is a networking scheme devised by Apple Computer for use in consumer electronics. It can now be found in TVs, stereos, home-theater setups, and enumerable other kinds of multimedia accessories.
FireWire has even served in automation equipment. Here it can work as an inexpensive means of networking between such devices as machine-vision cameras and industrial controllers. Automation suppliers such as Aerotech Inc., Ormec Systems Corp., National Instruments, and several others have even used FireWire to route commands and data among motor drives. FireWire networks have enough speed and an ability to carry deterministic communications that make such uses practical.
One drawback with using FireWire for automation, however, is that there is no common standard for the application programming that goes into industrial tasks. That basically means that automation vendors each devise their own methods for making equipment talk over FireWire. The FireWire equipment from one vendor won't talk to FireWire equipment from another.
This situation could change with the recent development of an open-source application layer for FireWire that targets industrial controls. The open standard is dubbed 1394AP for Application Protocol and essentially turns FireWire into a fieldbus. It is designed to allow the combination of motion, vision, and I/O from multiple vendors on a FireWire network.
The Fraunhofer Institute Photonic Microsystems in Germany originally devised 1394AP for a group of European automation suppliers. When the work was complete last year, Fraunhofer then released its code and specifications for the new standard to the 1394 Trade Association. Now interested parties can get a copy of the pertinent information (about 100 pages worth) by e-mailing a request to 1394 Trade Association President James Snider ([email protected]).
The 1394AP consists of an application layer that is an interface for bus management, and asynchronous and isochronous transfers used to control management, transaction, and link layer services. "For AP, we had to add a mechanism to deliver parameters in a way that the industry is familiar with," says Fraunhofer business unit manager, sensor/actuator systems Michael Scholles. "So the top level of the 1394AP protocol is now exactly the same as the widely used CAN (Controller Area Network) standard. A lot of people are familiar with CAN, and we wanted a protocol designed in such a way that application software needn't change much. So migration from a slow CAN bus to a fast FireWire bus goes quickly."
Scholles says the basic idea behind 1394AP is to exploit the isochronous feature of FireWire for use in industrial control. This mechanism makes it possible to send commands or parameters to devices (such as motors) on the bus at the same time with negligible jitter. The property of being isochronous is inherent in FireWire and there is no additional protocol needed to implement this feature.
Scholles thinks 1394AP stacks up well against alternatives such as real-time Ethernet. For one thing, certain industrial versions of Ethernet provide real-time capabilities only by assigning predefined time-slots to certain devices on the bus. There is no way to access a number of devices at exactly the same time as in FireWire.
He also points out that some variants of industrial Ethernet don't allow standard TCP/IP traffic to run on the same cable because to do so would disable the conditions needed for real-time capability. And 1394AP will be particularly useful for applications where industrial vision information coexists with control commands. 1394AP was designed so IIDC (the standard for sending uncompressed video over FireWire) can run simultaneously on the bus. The same can't be said for 100-Mbit Ethernet. It is, however, possible to run IIDC with camera inter-faces for the newer Gigabit Ethernet, though these currently are expensive.
Scholles says 1394AP technically resembles Profinet-IRT (Isochronous Real Time), a method promoted by controls maker Siemens. The problem with Profinet-IRT is that it runs only with a special ASIC for which Siemens is the sole source.
One caveat for 1394AP is that the finished standard has yet to be deployed in a real application. The original work on AP was a feasibility study commissioned by a group of European companies that included Basler Vision Technologies, Maxon Motor, and Nyquist Industrial Control. The intended use was in the area of industrial automation, but "We never really knew the exact application," says Scholles. "We were told it was for synchronizing motors as when running a newspaper printing press."
Fraunhofer also developed a controller to run the protocol as part of its study. The controller is a 16-bit chip from Infineon Technologies, also in Germany. Scholles says companies that were part of the original development group are building the protocol into products that are now on the drawing boards.
QUICK COORDINATION
Industrial controls makers that MACHINE DESIGN contacted say they haven't studied the 1394AP definitions. Nevertheless, there is some question about AP's interoperability goals. "We had to create a proprietary data packet for our Automation 3200 motion controller to get high-performance motion," says Robert Novotnak, advanced automation division manager at Aerotech Inc., Pittsburgh. "Right now, the hardware level of FireWire is the same or similar for all suppliers but each vendor has different methods of communicating to their amplifiers. When you generalize the communications protocol, you run the risk of losing the ability to get both high performance and functionality. We don't know how AP will pan out until we look closely at it."
Aerotech is among the motion-control vendors known for its use of FireWire networking. The firm's A3200 automation platform employs FireWire to minimize integration time and improve performance by eliminating many of the interconnect cables and breakout boards seen in ±10-V motion systems. In this distributed-control architecture, software running on an ordinary PC communicates over FireWire to Aerotech stepper modules and digital servomotor drives (called Ndrive, NPAQ, Nstep and NServo) to realize coordinated multiaxis motion.
Aerotech employed FireWire on its A3200 system, which has been commercially available for about five years, to take advantage of the network's speed and wide availability. "We wanted its 400-Mbit/sec transfer rate and we knew 1394b was coming which would push the bandwidth to 3.2 Gbit/sec over fiber. So there was a path for us," says Novotnak. "The determinism of 1394 was also a big factor in the decision to use FireWire. The contoured motion needed in laser processing, semi-conductor processing, or machine tools requires tight synchronization between the axes."
Aerotech equipment uses the full six-wire definition of FireWire. That includes two separately shielded twisted pairs for transmitting data, plus two power conductors. The idea behind the power leads is to keep the physical connection active as a safety feature. Aerotech equipment, for example, pulls power off the FireWire bus so if an amplifier in the system shuts down, its FireWire connection is still alive and the rest of the network can function without any chance of data corruption. (Some consumer equipment such as camcorders just use the four data wires alone to save space.)
The fact that power is available over the FireWire connection has some parties wondering whether the bus could be used in a manner analogous to Power Over Ethernet (POE) schemes. POE is most often deployed in connection with Internet-based telephone networks. But the power available over FireWire is limited to 1.5 A at 8 to 30 Vdc. That 45-W maximum is not enough to power even some industrial cameras and thus limits the potential applications for this feature.
CARS AND DRIVERS
FireWire seems to be alive and well in the auto industry. The first automotive audio systems to use FireWire came out last year. Says 1394 Trade Association President James Snider, "The model year 2009 and '10 cars will have 1394 networks for entertainment functions and digital video. You'll be able to plug your iPod into those cars rather than carry a glove box full of CDs around."
There was some speculation that Apple Computer was abandoning support for FireWire when it offered USB 2.0 cables on some of its iPod models. That turned out to be untrue — iPod users can substitute FireWire cable for USB, if need be. Moreover, Snider says use of the standard is growing, particularly in the audio/video industry. "1394 now is in about half the notebooks shipped today and in a little under half the PCs sold for home use, where it is generally used for connecting to a camcorder," he explains.
Home networking will make use of FireWire thanks to the advent of a long-distance version of the standard and its use in set-top boxes for high-definition TVs.
"FireWire would let one set-top box in the home give other sets access to TV signals rather than use multiple set-top boxes," explains Snider. He says at least one major home-automation supplier has plugs and panels in development for home networking with 1394. And there are also agreements in place to put the standard on the next generation of high-res DVD drives where the high bandwidth available in 1394b, the new long-distance version, will come in handy for transferring HDTV signals.
1394b recently became finalized and is available in ICs now reaching the market. "The problem with the first roll out was the 1394 implementation was a little basic; it wasn't everything we wanted it to be. So we are working with set-top box makers to come out with a more full-featured version," says Snider.
Inside 1394
The IEEE 1394 standard, aka FireWire or iWire, was originally created by Apple Computer as a high-speed way of interconnecting consumer electronics inexpensively. Up to 63 devices can be connected on a 1394 network. Bus bridges will extend that number to over 60,000. 1394 defines three signaling rates: 98.304, 196.608, and 393.216 Mbit/sec. Most industrial devices that use 1394 operate at the fastest rate. The latest version of the standard, 1394b, became finalized in 2002 and expands the definition to rates of 800 and 1,200 Mbit/sec. But devices of different speeds can operate on the same 1394 bus.
Modern PCs all support FireWire, though the 1394 Trade Association reports there have been problems running it with the most recent version of Windows XP. Regardless, 1394 is a peer-to-peer network so a device other than a PC can act as a bus controller.
1394 devices can connect in multiple configurations such as star or tree patterns with daisy-chain branches. In these networks, addressing takes place dynamically so there is no need for preaddressing.
1394 can multiplex various types of digital signals onto its two twisted-pair conductors. This makes it possible to pass video, audio, and device control commands over 1394. The standard also uses a fairness arbitration approach to assure all devices get access to the bus. The protocol includes device-specific commands for widely used consumer devices such as camcorders.
One reason for the interest in 1394 as an industrial bus is its isochronous data transmission. This mechanism guarantees a certain minimum data rate as needed for time-dependent data such as video, audio, or commands for fast servosystems.
Cables carrying 1394 signals are limited to 4.5 m between devices. Industrial suppliers that field 1394 equipment say the length restriction doesn't pose a problem because motor drives, amplifiers, and cameras tend to reside close together on most applications. Special transceivers have been developed to boost the length between devices used in home-automation systems.
MAKE CONTACT:
1394 Trade Association, 1394ta.org
Aerotech Inc., (412) 963-7470, aerotech.com
Fraunhofer IPMS, 49-351-8823-201, ipms.fraunhofer.de
Ormec Systems Corp., (585) 385-3520, ormec.com