On the way to plug and play
07 February 2008
Generic interface standards for modular embedded computer technology are moving on
Modularity has always been integral to embedded computer technology. Generic busses for adding application specific expansion cards to boards play a particularly important role. The most significant are ISA, PCI and the latest development, PCI Express (PCIe). As standards in the world of Office PCs, they are also commonplace on today’s embedded motherboards (ATX, Flex-ATX, MicroATX, Mini-ITX) and are related to PISA, PCI based systems and the first PICMG 1.x industry standards by identical electronic and physical interfaces.
However, the PICMG 2.x (CompactPCI) and PC/104 specifications have already broken away from the physical standards. Both form factors use the more robust press-fit connectors. The aim of both specifications was to raise the stability of intra-connects. Electronically, the signals have remained the same. The same is true for Computer-On-Modules. They use considerably more pins for the connections, but only because more interfaces need to be implemented via these connections. For example, ETX contains ISA and PCI along with various dedicated interfaces, and ETXexpress/COM Express offers manifold PCIe and PCI interfaces. But even here the electronics remains basically the same. The generic functionalities continue to be PC technology. They are simply adjusted to the needs of individual applications, as the following overview of embedded computing form factors shows.
Embedded form factors
In terms of standard interfaces, there is little difference between embedded motherboards and consumer motherboards. However, on some motherboards the expansion cards can be connected parallel to the board by means of riser connectors in order to reduce the height.
PICMG 1.x defines the SBC as a slot assembly. The expansion cards can be designed as permitted by the backplane. They are mostly classic PCIe, PCI or ISA cards that are also found in standard PCs.
With PISA, the slot SBC offers PCI and ISA on a half-length card via a double row of connectors on the backplane. Expansion cards are connected as with PICMG 1.x.
PC/104 plus defines stackable, almost square SBCs and expansion boards for PCI and ISA.
EPIC continues this tradition for I/O assemblies on a mini motherboard form factor. This is because the chipsets have more and more integrated functions and it makes sense to take advantage of them.
JRex/JFLEX has departed from the press-fit connector and ISA and supplies pure PCI and SMT equipped connectors. It also delivers a flexible concept that permits different expansion card sizes connected parallel to the SBC in order to save space.
Computer-On-Modules offer a range of interfaces to the baseboard. The respective standard (the most prevalent are DIMM-PC, ETX and COM Express/ETXexpress) defines the corresponding pin-out.
CompactPCI (PICMG 2.x) supplies PCI and switch fabric over the backplane. In addition there is hot-swap and, with IPMI (intelligent platform management interface), further hardware monitoring signals for enhanced system availability.
The ATCA standard (PICMG 3.x) and the new MicroTCA standard (PICMG MTCA.0); both have PCIe and Gigabit Ethernet over the backplane as well as hot-swap and IPMI.
True plug and play
These very different physical concepts have identical plug and play functionality to the PC bus technology. Originally, however, none of these form factors could be said to be plug and play. The term was introduced in the mid-90s with the arrival of Windows 95, but the implicit ideal of plug and play was still a long way off. There was no spontaneous connection between electronic devices or sub-systems that offered their services ad hoc on the network for the benefit of clients that could just as spontaneously call them up.
Today, standard expansion cards are inserted in systems running under today’s prevalent operating systems and automatically recognised by the SBC. The system automatically updates or installs the required driver and the functionality of the expansion card is available either directly or after a reboot. If the expansion cards have their own built-in user interfaces, users can set parameters according to their needs and (with the appropriate software) use the interfaces.
Peripheral devices
Peripheral devices also have a comparable plug and play functionality. For example, they can be connected via USB or intelligent clips, like the proprietary Wago I/O system that can be connected to the DIN rail mounted ThinkIO PC. The ATCA standard (PICMG 3.x) as well as the new MicroTCA standard define extended bandwidth with new, intelligent board-to-board interfaces and higher system availability that, for example, are based on PCIe (ATCA 3.4 / AMC.1) and offer Gigabit Ethernet switching (ATCA 3.1 / AMC.2) and other options for communication between boards. The differences between internal and external communication interfaces thereby become increasingly blurred and functions such as, for example, wake-on LAN and USB-boot are able to offer completely new plug and play options for systems.
How computer technology will develop in this area depends to a large extent upon what the providers of peripheral assemblies will offer. Internal expansion boards are currently undergoing a change from PCI to PCIe. On the one hand, this requires the development of new form factors or generations of form factors. It is closer to the ideal of plug and play hardware; through the separate, serial point-to-point connection of the periphery to the chipset as opposed to the parallel shared-bus-system of PCI, there are fewer data conflict problems.
Technology value
But how valuable are all these functions for embedded computer technology? An analogue or digital I/O card still doesn’t know automatically to which sensor or actuator it is connected, nor which data represents which parameters. And what about communications between two cards?
An embedded system gets its functions only once the different components have been empowered with intelligence by the individually developed application software. This requires developers to get to grips with drivers from different I/O suppliers, since the transfer of ordinary information such as, temperature, can have completely different storage locations depending upon the assembly. It would be a lot better if the I/Os had an abstract API. Even here, retrieving information on temperature can be very different depending on the board and can be connected with a different command, for example, ‘get temperature’ or ‘get temp’. This is why even the most sophisticated plug and play solutions for embedded computer technology, including UPnP, can never replace application specific programming. It is only possible to increase the degree of abstraction and thereby simplify application programming. This naturally applies to the embedded computer itself.
Companies like Kontron have been working to unify the different form factor APIs across all performance grades and to standardise the API form factor step by step. This will enable developers of embedded systems to directly supply the libraries for the different development systems as standard with the board, which will simplify software development and thereby reduce time to market. If every manufacturer were to supply this, including the reference programming, the amount of application programming for connecting different modules would reduce along with the screen design for applications.
Today, this is still a vision, but with increasing connectivity and the continued development of Internet technology it is already on the way to becoming reality. Interoperable plug and play will thus be within reach of embedded systems. The highly complex world of different systems will be reduced to the functional parameters of sensors and actuators, and to whatever programmers develop out of the interaction of the different plug and play components.
NORBERT HAUSER is vice president of marketing, Kontron AG
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