The Evolution of the Embedded PC

by Michael Peat, INSIDE Technology USA, Inc.

As users of embedded technologies continue to demand more and more features, embedded PC manufacturers are taking greater advantage of advanced PC technology. In the beginning, manufacturers of embedded computers tended to bring PC technology to existing bus structures. In the mid to late 1980s, for instance, companies like Ziatech (San Louis Obispo, CA), <%=company%> (Arlington, TX) and Pro-Log (Monterey, CA), to name a few, created single board PC compatible computers for the already existing STD Bus.

The STD Bus was created in the late 1970s as a standardized, 8 bit, modular architecture based on the Z80 processor and peripherals. Because the PC/XT bus was so similar to the STD Bus, it was a natural progression to bring PC compatibility to this architecture, which already had a fairly large installed base. The advent of PC compatibility on the STD Bus brought with it the use of MS-DOS, and all of the software development tools that were (and are) available for the PC. This was a great boon to the market, and it probably extended the life of the STD Bus by at least five years.

However, with the migration to 16 bit and then 32 bit processors, the STD Bus was stretched to its performance limits. After all, it was designed as an 8 bit bus. Several manufacturers implemented different schemes to increase the size of the data bus to 16 and 32 bits. The method adopted by the STD Bus Manufacturer's Group latched a portion of the address bus and multiplexed the upper byte of the 16 bit transfer across the dormant address lines. Of course, this method topped out at 16 bit across the bus transfers, and many users were not comfortable with the multiplexing scheme. The other method involved adding an interleaved row of contacts to the existing STD Bus card edge, which provided the additional pins for data transfer, address lines and control signals. This scheme, proposed by Ziatech, was dubbed "STD-32", and was adopted by several STD Bus manufacturers. Products for both schemes are still available today from a number of vendors.

As PC technology evolved, several new standards began to carve their place in the marketplace. Several companies developed ISA bus single board computers and peripherals to be used in passive back planes. This approach gave the system designer the opportunity to make use of existing standard connectors, while also creating a more durable system overall, when compared to "desktop PC" hardware of similar functionality. Since one of the primary goals of the embedded PC designer is size reduction, the passive back plane approach is also appealing because it is smaller than conventional PC "motherboard" architecture. The durability, smaller size, and relative low cost of the passive backplane approach make it very appealing for many different OEM applications. These products are used in factory floor control, GPS based moving map applications for military and commercial vehicles, small kiosks, embedded web servers accessed via the internet, and just about anywhere that a small, highly integrated PC is needed. N/APC/104 Comes Of Age
In the continuing effort to reduce overall system size, the PC/104 standard was proposed by <%=company%> (San Jose, CA) in the early 1990s. The 3.6 X 3.8 inch form factor has made this the preferred embedded PC architecture in applications driven primarily by smaller size requirements. Because of PC/104's lack of real estate, it is difficult to fit a lot of functionality on a single PCB. For this reason, PC/104 modules are stackable, allowing the designer to add modules in accordance with the demands of a given system design. For instance, there are display adapters for flat panel devices, data acquisition cards, digital I/O cards, GPS receivers, and many other functions available from literally over 100 different manufacturers.

PC/104 is also used as a mezzanine expansion path for larger single board computers. Many manufacturers of VME, STD, ISA, and other bus architecture based SBCs implement PC/104 as an inexpensive method to add additional features to a given system design. The fact that PC/104 is essentially the ISA bus electrically has made it relatively easy to develop I/O cards using off the shelf chips intended for the desktop and notebook PC market.

As customer demand for increased performance has developed, the PC/104 Consortium proposed bringing the PCI bus to the PC/104 architecture in the form of PC/104+, which adds an additional connector to implement the PCI bus in the existing PC/104 form factor. The caveat is, and this is no small consideration, the lack of space on the 3.6 X 3.8-in. architecture, makes it very difficult for manufacturers to develop single board computers within the standard form factor because of the size of the PCI connector. Because of this, PC/104+ is used almost exclusively as mezzanine bus for the expansion of larger form-factor single board computers in applications where high performance peripherals are required. For instance, there are frame grabbers, high-end graphics controllers and high speed network controllers available now for PC/104+. N/AMore Horsepower!
As other bus architectures moved to increase their bandwidth and functionality, manufacturers of passive backplane PC compatible systems banded together to form the PCI Industrial Computer Manufacturer's Group (PICMG) which is responsible for promoting and maintaining two standards.

The CompactPCI standard, developed and initially proposed by Ziatech Corporation (San Louis Obispo, CA) is a pin and socket interconnect passive backplane standard. The CompactPCI form factor is very similar to that of the VME bus, in both 3U and 6U board sizes. Because a card cage is required, CompactPCI is a fairly expensive solution; however, it is intended for very high-end applications where high data throughput is required. For this reason, the extra cost is easily justified. There is a great deal of anecdotal evidence to suggest that many users that have traditionally used the high performance VME bus are migrating to CompactPCI. In fact, many of the present CompactPCI vendors have historically been VME bus manufacturers.

The other standard that is maintained by the PICMG is the ISA/PCI passive backplane standard. ISA/PCI essentially allows for single board computers with both the card-edge ISA bus interface, as well as a card-edge PCI connector in an "in-line" configuration. These products are meant for use in a passive backplane, which has both ISA bus expansion slots as well as PCI bus expansion slots.

While intended for high reliability industrial applications, and perfectly capable of performing well on the factory floor and other industrial applications, the ISA/PCI mechanical interconnect scheme is not as rugged as CompactPCI. Therefore, overlap between the ISA/PCI and CompactPCI target markets is negligible. The two approaches provide the same level of performance, while CompactPCI targets extremely harsh environments where the primary concern is the ruggedness of the system design.

On the other hand, ISA/PCI single board computers are quite capable of handling environmental extremes beyond those of desktop PCs. In fact, they are being used in vehicle and other applications where vibration and temperature exceed the limits a desktop PC would be capable of tolerating. For those applications that need a high performance "industrial computer" but are limited by cost concerns, the ISA/PCI scheme addresses both the economic and bandwidth aspects of the designer's system requirements. N/AWhat Is To Come?
An industry insider was once heard remarking, "Embedded PC technology tends to trail behind the emergence of, for lack of a better word, 'conventional' PC technology by 1 to 1.5 years." Our observation has been that this is generally true. That being the case, it is possible to predict general technology trends by keeping an eye on the state of the art of desktop PC technology.

As an example of this phenomenon, one need only look at the new offerings from the major PC manufacturers and compare them to those of the embedded PC manufacturers. While the very high end of the desktop market is presently employing 333 MHz PentiumII processors, the embedded marketplace is just now moving to the higher end Pentium (> 200 MHz) processors, and has only very recently added support for MMX capable processors. One can certainly envision that many an embedded PC design engineer has a P2 design on the drawing board (or CAE workstation) and we will probably see these begin to roll out as 1998 progresses.

As for new bus architectures, well, that is one of the hardest things to predict. Fortunately, there has been considerable forethought within the development of the PCI architecture, and, hence, it will probably be the platform of choice for the foreseeable future. This, of course, leaves plenty of room for new innovations within the framework ISA/PCI, CompactPCI, and PC/104 Plus standards.

High bandwidth connectivity seems to be the latest focus of many manufacturers of Embedded PC products. Embedded PC vendors in ever increasing numbers are adopting USB, 100Base-T Ethernet, and "Firewire" as well as infrared interface (IrDA), and we can expect to see a continuing migration toward other connectivity innovations that are now making there way onto desktops in homes and businesses in the coming year.

Embedded PCs are everywhere, and innovations in connectivity will allow for things like embedded web servers, demanding high-speed networked control systems, as well as the incorporation of high-speed peripheral devices. One can only imagine the future role of the embedded PC, but as long as customers continue to make greater demands on embedded PC vendors, innovation will certainly bring new and exciting products to overcome the challenges that this kind of interaction invariably create. The only certainty is that the embedded PC will be with us well into the next century.

INSIDE Technology USA, Inc., 8 Prestige Circle, Suite 116, Allen, Texas 75002. Tel: (972) 390-8593; Fax: (972) 390-8609.