The various transmission options with USB 3.0

After four years, the USB 3.0 interface has successfully established itself in the machine vision market. The USB3 standard plays an important role in this success.

Para los cables, el USB3 Vision Standard sólo define las especificaciones mecánicas de los conectores para la interfaz USB 3.0. Además, desde la introducción del USB 3.0, ha persistido el rumor de que esta interfaz está limitada a una longitud máxima de cable de 3 metros. Sin embargo, esta limitación sólo se aplica a los cables pasivos que están estrictamente diseñados de acuerdo con el estándar general USB. La experiencia demuestra que las especificaciones del estándar USB son demasiado estrictas para el mercado de visión artificial. Con cables pasivos, USB 3.0 permite longitudes de cable de hasta 8 metros. También existen soluciones activas y ópticas para la transmisión de datos a través de USB 3.0 para distancias más largas. Aquí explicamos las diferentes opciones de transmisión con USB 3.0 y hacemos las pertinentens recomendaciones para diferentes tipos de aplicaciones.

For the cables, the USB3 Vision Standard only defines the mechanical specifications of the connectors for the USB 3.0 interface. In addition, since the introduction of USB 3.0, there has been a rumour that this interface is limited to a maximum cable length of 3 metres. However, this limitation only applies to passive cables that are strictly designed in accordance with the general USB standard. Experience shows that the specifications of the USB standard are too strict for the artificial vision market. With passive cables, USB 3.0 allows cable lengths of up to 8 meters. There are also active and optical solutions for data transmission via USB 3.0 for longer distances. Here we explain the different transmission options with USB 3.0 and make recommendations for different types of applications.

Passive data transmission

While twisted pair cables are less susceptible to errors, Twinax cables are suitable for longer transmission distances. When contacting and connecting the plugs to the raw cable, it is important to have only low impedance discontinuities in the transition area. In addition, the conductor cross-sections must have adequate dimensions. So, it is important to make sure that the core material is not contaminated.

Tolerance limits that are too low can cause system instability, such as image loss.

Data transmission active

The computer does not detect the redriver as a participant between the camera and the host. Signal preparation is not perceived as an additional interpretation step. Booster converters, which artificially increase the voltage of the electrical signal, can be installed in addition to the redrivers. The step-by-step converters consume additional electricity, but also guarantee the necessary 5 V power supply on the side.

However, the designs of the active cable products that were previously available on the market have not yet fully matured.

Optical data transmission

With higher voltage and thinner passive cores, it is possible to implement longer transmission paths even for the power supply without large losses. Alternatively, the voltage can also be supplied at the end of the transmission path independently of the optical cable so that the maximum transmission length of the fiber optic cable can be used.

Recommendations for using USB 3.0

Due to the properties of the different transmission options, the following lengths are recommended for fixed installation of USB 3.0 data cables:

USB 3.0- cables de datos
Longitud 0,1 m – 5 m > 5 m – 8 m > 8 m – 20 m > 20 m
Tecnología Pasivo:
Par Trenzado
Pasivo:
Twinax
Optical:
Hibrido
Optical:
Puramente óptico
Aspectos
Esenciales
– Calidad Industrialmente adecuada
– Sistema de cámaras de grandes dimensiones
– Fuente de tensión de grandes dimensiones
( 5 V / 900 mA )

Experience shows that the active components of the USB cables are not yet technically mature for data transmission in the image processing market and only allow the secure connection of two 4-meter passive USB 3.0 data cables in total.

Therefore, USB 3.0 data cables can only be recommended if cable interruption is unavoidable, if there is a clean design for a redriver and if no passive Twinax cable can be used.

USB 3.0, Gigabit Ethernet and Camera Link

With the introduction of the USB3 Vision Standard, USB 3.0 was created as a valuable transmission tool in addition to Gigabit Ethernet and Camera Link. The advantages of USB 3.0 are a bandwidth of up to 380 MB/s, low integration costs thanks to the plug-and-play system and the possibility of achieving cable lengths of more than 8 metres using various transmission techniques. Gigabit Ethernet, on the other hand, offers the advantages of successfully integrating multi-camera systems and enabling long cables of up to 100 meters with a low bandwidth of up to 100 MB/s. Poor quality USB 3.0 cables can cause error messages such as “lost images” or completely dropped connections between the camera and the host.

This may create the impression that the USB 3.0 system is unreliable or unstable.

Summary

The USB 3.0 interface offers several transmission options in the form of passive, active and optical data transmission. A recommendation for the use of the different technologies is based on the necessary length of the data cable. With the right cable and USB 3.0 accessories, passive data transmission can be guaranteed for lengths of up to 8 meters. Optical or hybrid cables also make the USB 3.0 interface suitable for data transmissions well beyond 8 meters; it has already established itself as an indispensable data interface in the machine vision market. With its easy activation, high bandwidth and cost efficiency of the entire camera system, the USB 3.0 data interface bridges the gap between Gigabit Ethernet and Camera Link.

Wider cores reduce the damping of data transmissions.

Passive data cables can lead users to mistakenly believe that cores with higher cross-sections can reduce the damping of data transmissions. This is only true to a certain extent. At higher transmission frequencies, the so-called skin effect occurs: The behaviour of currents in a conductor depends on the frequency. The higher the frequency, the higher the current concentration outside the conductor. As a result, from a certain frequency, the current is transferred mainly at the edge of the cross section. Due to the skin effect, the centre of a core with a larger cross-section no longer contributes significantly to the conductivity of the conductor.

Fiber optic cables are much more robust than their reputation suggests.

Contrary to many assumptions, fibre optic cables are optimally protected, have minimal bending radii and are resistant to EMC. Therefore, in a fixed installation, they meet the same standards as passive copper cables. With the right design, they can also be used in mobile applications.

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