FireWire IEEE 1394 a / b
FireWire, also known as i.Link or IEEE 1394, is a serial bus system developed by Apple as successor of the SCSII bus. The brand name FireWire, to be found only on Macintosh computers by Apple for a long time, became popular only after the introduction of the i.Link interface by Sony as part of the IEEE 1394a standard for consumer products, such as camcorders. This 1394a standard, introduced in 2000, allows for a fast data exchange between computer and multimedia or other peripheral devices. FireWire now competes with the widely used USB 2.0 bus, however, it had a data rate of 400 Mbit/s already years before.
Advantages of FireWire IEEE 1394 a/b over other analogue systems
Advantages of FireWire IEEE 1394 a/b over other analogue systems
- The digital image transmission provides a loss-free signal transfer and therefore a better image than analogue image transfer.
- Omission of the expensive frame grabber card. Many PCs already provide an integrated FireWire interface. (Nevertheless it is recommendable for machine vision to use an IEEE 1394 PCI card which, however, is cheap compared to a frame grabber.)
- Cheap standardised cable and plug technology. Even high-quality robust FireWire cables with lockable plus are clearly cheaper than analogue Hirose cables (simple analogue coaxial cables aside...).
- Hot plug, peer-to-peer communication, integrated voltage supply in the cable provide for a sophisticated operation even of several cameras on the bus.
Characteristics of FireWire 400 - IEEE 1394
- Transmission bandwidth of 100, 200 or 400 Mbit/s (12, 25 or 50 megabytes per second gross); therefore image data of approximately 38 MB per second net can be transported in camera operation.
- The technology offers hot plug / hot unplug: devices can be connected and detected during operation.
- Integrated voltage supply (8 to 33 VDC, 1.5 A, max. 48 W) for devices in the 6-pole cable. FireWire 400 uses a 6-pole cable, four data lines and two lines for bus voltage. The 4-pole variant ("i.Link" by Sony) provides no voltage supply and is therefore not used for industrial machine vision cameras.
- The thin flexible 6-pole cables are shielded twisted pairs (STP), quite interference-free, have a thin cross-section and can easily be laid in an industrial environment.
- Data transmission is possible in both directions: camera control, triggering, parameterisation is equally possible during operation.
- Officially specified are 4.5 m maximum distance between two devices (at 400 Mbit/s), yet up to a 10 m can be realised in practice. If the speed is limited to 200 Mbit/s, a maximum of 14 m can be bridged. The maximum transmission length strongly depends on the quality of the integrated bus components in camera and PC and the quality of the cable.
- Longer distances require inline repeaters or FireWire hubs which are connected in between.
- In theory, up to 63 devices can operated on one bus, 16 of which can be cameras. The term "daisy chain" refers to a wiring pattern in which one device is connected to the following one; i.e. A to B, B to C, C to D. IEEE 1394 devices usually have two connection in order that the mentioned chaining can be realised. Cable lengths can be maximised in this way.
- Practical experience with IEEE camera systems in industrial facilities, however, reveals that up to 3 cameras on one bus always work in a stable and reasonable manner, yet interferences, problems with the overall cable length, etc. occur beyond that. Therefore use rather one or several dual FireWire cards if you want to operate more cameras, particularly synchronously and with high load.
Characteristics of FireWire 800 - IEEE 1394b
- A different signal coding and signal level serves to achieve a transmission bandwidth of 800 Mbit/s (100 megabytes per second); therefore image data of approximately 75 MB per second net can be transported in camera operation.
- The cable has been changed, now it has nine conductors and a new plug, but a backwards compatibility to 1394a was made possible using bilingual controller chips. A FireWire 1394b interface can operate FireWire 1394a devices, too, using suitable adapter cables.
- FireWire 1394b allows for the use of different cable materials, e.g. fibreglass. Therefore cable lengths of up to 100 m are no longer a problem. Modern camera suppliers offer appropriate cameras which cannot only connect copper cables but also fibre optic cables.
The successor IEEE 1394b is much faster than USB 2.0 (approx. 40 MB/s net), having an effective net image transmission bandwidth of approximately 75 MB/s.
The DCAM standard and IIDC
DCAM stands for "1394-based Digital Camera Specification" and specifies the behaviour of 1394-devices with uncompressed image data transmission (without audio). The DCAM standard is only supported by devices which provide a FireWire (IEEE 1394) interface. The responsible working group is the IIDC (Instrumentation and Industrial Control Working Group), thus the standard is called IIDC in newer specifications. Usually these are FireWire cameras with industrial features which are used for inspection applications, for instance. The DCAM standard also defines the standard resolution and image refresh rates of the cameras in different formats. A special role for industrial cameras, however, plays "format 7" which allows for random resolutions and frame rates.
It does not only define the structure of the image data stream generated by the camera, but also the basic parameterisation of the camera (e.g. exposure time, gain, white balance, etc.) and how the camera is to be parameterised.
The PC must support DCAM in order to communicate with a DCAM camera. For this purpose the operating system requires a DCAM driver which is already included in Windows 2000, XP and Vista. However it only supports the most important camera parameters.
Special drivers supplied by the individual camera manufacturers provide a remedy, who are of course interested in setting themselves apart from their competitors by means of special camera features (LUT, shading correction, special trigger modes, etc.). Only by using these drivers the full range of functions and performance of the hardware can be used, yet from this moment on, the camera system is proprietary and bound to one particular manufacturer. The Gigabit Ethernet / GenICam standard can provide a solution.
Neue Kameramodelle werden nicht mehr für diese Schnittstelle entwickelt, Kameras mit GigE Vision oder USB3 Vision Interface ersetzen zunehmend diese Geräte. Conclusion:
Camera families on the basis of FireWire a/b are available today which cover almost all fields of application from 0.3 to 16 megapixels, monochrome or colour, CCD or CMOS. Unfortunately, despite the DCAM/IIDC standard, cameras of different manufacturers are not really 100% plug&play-compatible, special features are only mapped in manufacturer-specific drivers. Without the use of fibre optic transmission, the cable lengths are limited, too.
New camera models are no longer being developed for this interface, cameras with GigE Vision or USB3.x Vision interface are increasingly replacing these devices.