26 April 2010
Driving over distances: Industrial communications
How an automotive communication protocol can be applied in the industrial sector.
Industrial applications have always required an easy interface for the operator. But the traditional buttons and control sticks, combined with small information panels, are being replaced or extended with digital displays. When equipped with a touchscreen, displays allow for dynamic screen content and act as an intuitive operator interface. With pc functionality available in small, power saving modules, control units can easily handle digital user interfaces.
Unlike office applications, the user interface for industrial automation applications is often located remotely from the controlling pc. This requires expensive special cabling or the use of multiple pcs connected via Ethernet, for each panel. There is an obvious need for a low cost, robust, high speed, full duplex link which meets the requirements of the industrial environment for electromagnetic interference (emi) and distance.
The Automotive Pixel Link (APIX) technology combines high speed digital video transfer with full duplex, two wire data communication. APIX industrial offers standardised connection of remote displays over single twisted pair cables without sacrificing the flexibility necessary to meet the application specific requirements.
APIX technology, which defines a physical layer for display and camera links, was originally created for the automotive sector, where emi is a major requirement.
With a bandwidth of 1Gbit/s, the APIX link supports video resolutions of 800 x 600 with a colour depth of 18bit. The high speed downstream channel acts as a transparent gateway for the parallel video interface, providing the data is sampled at the graphics processor interface at the same clock at the display. Video data is transmitted in real time without compression, minimising latency in time critical applications and eliminating the need for decompression at the display.
In addition to a video interface, the APIX architecture offers sideband data channels, which provide full duplex downstream and upstream capabilities. Sideband data is sampled at either transmitter or receiver and is provided transparently at the respective pins at the remote site. Since the devices do not require special formatting, this transparent sampling enables the flexible 'extension' of interfaces like uart or spi over long distance.
The differential link is independent from the video pixel clock and therefore allows the transmission of the sideband data, even without an available pixel clock. The ability to carry full duplex information without a pixel interface offers significant benefits for point to point applications. Since the sideband data channel is available as soon as the link is powered up, it can be used as the main configuration interface for the camera or display implementation. And as downstream and upstream data is transmitted over high speed differential serial lines, it offers the same distance and emi performance as the video link.
An APIX link requires a maximum of two pairs of shielded twisted wires, transporting the 'downstream' video and data stream over one pair and the 'upstream' on the other.
Due to ac coupling of the APIX physical layer, the data lines can also be used for supplying a remote system. The power supply is realised by using either one or both existing wire pairs, with inductors acting as low pass filters to the signal lines. Since two pairs of wires are typically used, it is recommended to use the high speed downstream pair for power and the other for ground. In this way, a display can be supplied with video, data communication and power using two pairs of wires.
The APIX link, as any other differential technology, requires a twisted pair cable to ensure the positive and negative aspects of the differential signal pair negate as optimally as possible.
Standard CAT5 Ethernet cable, which is available as shielded or unshielded version, offers four differential pairs, enabling the transmission of APIX downstream and upstream.
While CAT5 cables are available from many vendors, their quality may vary, which affects maximum transmission length. For distances of more than 15m, it is recommended that cables be testd. For extended distances, it is possible to use cables which are optimised for differential signalling of two pairs. Examples include star-quad topology cables, offering high crosstalk attenuation at a very compact cable diameter.
The selection of connectors and plugs is equally important. One of the most common connectors for Ethernet applications is the RJ45 connector, which is used with CAT5 cables and is available as plastic version or surrounded by a metal case for better EMI shielding. RJ45 connectors are specified with high contact durability, which allows many different and flexible installations.
Alternatively, a more rugged, but more expensive connector, can be used. The Rosenberger HSD connector is widely used for automotive applications. The HSD concept is optimised for high robustness in terms of connector/plug stability. The connectors are delivered already assembled with the cable to guarantee maximum signal quality. The HSD connectors are used with star-quad cables, carrying the downstream and upstream of the APIX link while still allowing to transmit power supply over the same cable.
Standard connector pin outs have been defined to support the independent development of display and graphics units in the industry.
The basic RJ45 connector pin out offers the following possibilities
• High speed video link
• Robust communication channel (sideband)
• Power supply over the data lines
• Additional power supply capability
• Two additional lines for synchronous data transfer (like display DDC data)
In order to enable quick and easy evaluation of the APIX Display Interface, congatec has created the APIX Design Kit. It contains a PCI Express add on card including operating system drivers and a remote display unit with integrated touch screen. The design kit also features comprehensive documentation, complete with schematics, to allow implementation of customised solutions.
The APIX PCI Express card features two independent APIX interfaces via RJ45 connectors. This allows for the operation of two independent display units through the use of conventional CAT5 cables. When using multiple cards in a system, up to eight APIX channels can be operated in parallel.
The sideband signals of the APIX channels offer the possibility to transmit UART signals, which can be used for touch screen or keyboard applications. More complex or encrypted communication can be accomplished using the Fujitsu's MB88F332 (Indigo) graphic display controller as a receiver at the display unit side.
The corresponding graphic data and protocols are processed by a PCI Express connected fpga, which passes RGB data to the APIX transmitters. Physical transfer is handled by the APIX channels and power transmission to the display units is configurable. The APIX card supports PoA, but can also provide 3.3V power from the PCI Express interface or can transmit external 5V or 12V power sources. The congatec Dual APIX PCI Express Card is a plug and play card and includes all necessary operating system drivers.
APIX provides a solution for low cost digital remote user interfaces in industrial applications. With the congatec starter kit, it is possible to develop innovative solutions quickly and easily.
Author profile:
Christian Eder is congatec's marketing manager
Author
Christian Eder
Supporting Information
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24570\P31-32.pdf
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http://www.congatec.com
Companies
congatec AG
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