Since their introduction in 1991, PC/104 standard based systems have been widely adopted in various applications. Unmanned aircraft control, onboard vehicle control and navigation systems, personal communicators, add-on cards for customer specific boards, all utilize PC/104 technology. Many system developers choose this form-factor due to its advantages in terms of low weight, compact size (boards are just 90×96 mm), ease of application development and modification, and mechanical reliability.
According to the Electronics Trend Publications data, the global PC/104 market amounts to around $170 million US per year. Statistics show the largest volumes of PC/104 boards are used in industrial and defense systems (50% and 20% respectively). Functionally PC/104 boards are divided into two categories, practically identical in terms of sales volume: CPU (or processor) boards and input-output boards.
Since reliability of the entire system depends on the reliability of its components, system development and the selection of each component (PCBs, connectors, chassis, power supplies, heatsinks etc.) should be done thoroughly, with great attention to details.
Extreme Temperatures and Computing Performance
Many applications require PC/104 systems to operate within a wide temperature range that often exceeds the manufacturers’ recommendations. To considerably increase reliability and widen the operating temperature range, one can choose a more robust version of a certain component type – for example, use tantalum capacitors instead of electrolytic ones. Unfortunately there is no such option for other components such as high-performance CPUs, chipsets, and memory. Therefore, when developing a board for harsh operating conditions, the key tasks are performing a thermal effects analysis, building the board to withstand the worst possible working conditions, and thoroughly and comprehensively testing the product within the entire operating temperature range.
Due to the boards’ small size and far from ideal heat dissipation conditions inside the closed cabinet, special attention should be paid to thermal stability and functionality, not only at low operating temperatures, but especially at high ambient temperatures. Forced air-cooling is not always acceptable in these systems because of their very compact size. Cooling fans must also be more robust than normal. The MTBF of an average fan does not normally exceed 20,000 hours, but PC/104 processor boards itself should be designed to have an MTBF in excess of 100,000 hours.
In their effort to cope with the low heat dissipation capability of PC/104 systems, many processor board manufacturers use low heat emitting CPUs, such as Pentium III operating at 300-600 MHz with reduced power consumption. However, these processors have relatively low performance compared to what is needed for modern applications. Moreover, Intel recently announced the phase-out of ultra low voltage Pentium III processors, so it is risky to count on what is left on suppliers’ stocks. It is more reasonable to use low voltage versions of Intel Pentium M family processors. These CPUs are manufactured using special technology and have almost doubled the advantage in frequency-to-TDP ratio compared to other processors. The table below presents frequency and TDP data for standard processors, their low voltage versions for embedded applications, and chipsets.
Even when developers select a low thermal power processor for their application, they still have to solve the task of heat dissipation from the CPU and GMCH. Only a few companies in the world offer PC/104-Plus processor boards with CPUs 1GHz and up.
Common practice among PC/104 manufacturers is to locate the CPU on the PCI and ISA connectors’ side of the board. These connectors and any expansion boards placed on the processor board prevent effective heat dissipation from the CPU. Fastwel’s PC/104-Plus processor boards, however, are designed to either be on top or bottom of the stack of PC/104-Plus cards, and the CPU itself is placed on the opposite side of the PC/104-Plus connector. Thus, the task of heat sinking is significantly simplified since heat can be drawn from the CPU and GMCH chip via a heat conductive plate to the cabinet. In such cases the contact area is much bigger and the overall heat transmission resistance is much lower than in those boards where heat pipes are used to bring the heat along the board surface to the sides of PC/104 enclosure. Provision of a low heat-resistance thermal bridge between CPU and system chassis allows use of the whole PC/104 cabinet as one large heatsink.
This solution offers advantages in size, weight, price and overall system design. It also allows the use of high performance Pentium M processors operating at frequencies of up to 2 GHz with 533 MHz front side bus. For applications with high data exchange between CPU, memory and I/O, the limitation in the system bus can be crucial; that is why the increase of system bus frequency from 400 to 533 MHz can give a 30% benefit in certain system performance.
Shock, Vibration and Harsh Environment Resistance
Systems used in transportation or industrial environments are very often exposed to constant vibration loads. In these applications, the PC/104 architecture is a good choice due to reliable fastening, small size, and low weight of the PCBs, as well as reliability of the PCI and ISA connectors. Reliability may be further increased by soldering normally socketed components. The use of a soldered CPU is routine technology, whereas soldering DRAM chips leads to parallel circuitry routing and requires the addition of several conducting PCB layers. Moreover, soldered DRAM chips providing 1 GB of memory consume about 15% of the PC/104-Plus board’s usable surface area. Soldering of components also considerably complicates design and manufacturing, but drastically improves shock and vibration resistance. For example Fastwel’s CPC1600 can withstand vibration levels up to 5G within the 10 to 500 Hz frequency range, whereas SBCs with SODIMM memory usually will only withstand tests up to 2G.
There is one more “side effect” of having all the components soldered. Efficiency of additional damp-proof board coating increases when more components are soldered, bringing down the risk of short-circuit caused not only by the condensed moisture, but also by salty mist, metallic particles or corrosion.
Expansion Interfaces and Data Input-Output
Early on PC/104 cards were equipped with only an ISA connector for plugging in extension modules. At that time the number “104” corresponded to the number of contacts between the connected modules. In 1997 the PC/104 consortium approved a new PC/104-Plus specification introducing an additional 120-contact connector provided for PCI interface extension modules. Unlike the PCI bus standard connector having 124 contacts, the PC/104-Plus PCI bus does not support 64-bit data transfer. PC/104-Plus compatible systems are designed to support up to four extension cards via this bus. Maximum theoretic bandwidth of the PCI bus within the PC/104 architecture amounts to 132 MB/s, while the real throughput does not exceed 55 MB/s.
The main applications using PCI bus within the PC/104 framework are: extension boards with Ethernet controllers, video capture modules, digital signal processing boards and other applications requiring high speed data exchange rate for the CPU. However for many modern applications the bandwidth capacity of the 32-bit PCI bus is not enough. For example, many graphics cards require 500 MB/s and even more. Video-encoding and recording tasks need more capacity. One channel MPEG-2 video compression solution using the Philips Semiconductors SAA6752 chip requires ~8 MB/s. Hence, ~5 video channels use the whole bandwidth capacity of the PCI bus.
The most bandwidth demanding applications can be realized by means of a PCI Express bus. Being serial, PCI Express has a carrier frequency of 2.5 GHz and can provide up to 2.5 Gb/s per one x1 lane with the option to combine lanes in various configurations like x4, x8 and x16. In addition to high throughput, other advantages of PCI Express include: lower signal delay values, improved data burst transfer protocol, and the option to set processing priority for data packages (Quality-of-Service).
At the physical level one PCI Express channel is realized as two pairs of low voltage differential signal lines at 2.5 GHz. This feature offers a significant advantage for size-constrained boards like PC/104. Less routing means less interconnect layers in PCB and more space for additional components on the board.
PCI Express can provide advantages not only for onboard data transfer, but for board-to-board connection as well. For example, most modern Intel server boards do not have PCI slots anymore; most expansion boards now work via PCI Express interfaces. From a global perspective the move from PCI to PCI Express looks like a logical step in the overall shift from parallel interfaces to serial ones. Thus, USB is replacing LPT, and SATA is overcoming EIDE.
The nearest “neighbor” to the PC/104 standard, the EPIC standard has recently received the pre-released version of EPIC-Express specification. Single board computers manufactured according to EPIC standard can accommodate PC/104 modules through the same ISA and PCI connectors. More information on EPIC-Express can be found at www.epic-express.org.
In the EPIC-Express specification, the PCI bus is replaced by a PCI Express bus with either 4 - 10 Gb/s (one bank of connectors) or 12 Gb/s (three banks of connectors) PCI-Express lanes.
This solution is quite logical, because the bus used for data exchange (PCI) is replaced by a faster one (PCI-Express), while a low speed ISA bus, traditionally used for simple I/O tasks and for signaling is kept unchanged.
Thanks to standards such as PCI Express and EPIC-Express, embedded system developers can now use new compact boards equipped with high performance CPUs and high-speed serial data exchange interfaces. Security video processing (down-steaming, encoding, packetizing and storing), image capturing and recognition in surveillance systems, target capturing and tracking in defense applications are just a few of the possible applications for such high performance computing boards. These new products open new horizons in development of modern high-performance solutions for robotics, security, transportation, avionics and defense systems.