On the way to plug & play functionality
17 September 2008
Modularity has always been integral to embedded computer technology. Generic buses, i.e ISA, PCI and PCI Express (PCIe) for adding application specific expansion cards to boards play an important role
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 COM (computer on modules). They use more pins for the connections 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. However, the electronics remain, basically, the same. The generic functionalities continue to be PC technology. They can be adjusted for individual applications. 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.
Definitions
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. The chipsets have more integrated functions.
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. COMs 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 pinout.
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.
Plug & play functionality
These different physical concepts have identical plug & play functionality to the PC bus technology. Originally, however, none of these form factors could be said to be plug & play. 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. 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 also have a comparable plug & 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 become increasingly blurred and functions such as, for example, Wake-On LAN and USB-boot are able to offer new plug & play options for systems.
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. On the other, it gets closer to the ideal of plug & 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. In terms of hardware control, plug & play is quite well advanced.
An analogue or digital I/O card still does not know automatically to which sensor or actuator it is connected, nor which data represents which parameters. And what about communication 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, for example,
‘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, but even here, retrieving information on temperature can be different depending on the board and connected with a different command, for example, ‘get temperature’ or ‘get temp’.
Even the most sophisticated plug & 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. Kontron has 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 & 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 & play components.
Author: Norbert Hauser is Vice President of Marketing at Kontron AG
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