Embedded Architecture
PC/104: Embedded Technology for Medical Electronics Development
PC/104 architecture provides compact design and durability to meet the requirements of medical device manufacturers.
Mike Southworth
From infant respirators to linear accelerators, a growing number of today's medical devices use PC/104 embedded modules as their underlying board-level architecture. These modules allow manufacturers to overcome traditional design hurdles and bring products to market quickly and efficiently. Based on desktop PC technology, yet packaged in a smaller, more-durable form factor, PC/104 provides design engineers with modular building blocks that are prequalified and cost-effective for medical applications.
For medical equipment manufacturers, PC/104 offers distinct advantages: compact size, PC compatibility, strong vendor support, stackable design, low power requirements, environmental durability, and simple maintenance.
Compact, PC-Compatible Design
A smaller, repackaged version of the industry standard architecture
(ISA) bus board, a PC/104 module measures only 3.6 x 3.8 in. (90 x 96 mm)—comparable
to a 3.5-in. floppy diskette. Deriving the name PC/104 from its IBM personal computer
(PC) architecture and 104-pin connector, PC/104 circuit boards provide complete
PC/AT compatibility without the large PC card form factor (12.5 x 4.8 in.). The
same internal definitions of a desktop PC are incorporated into PC/104's specification
(IEEE Draft P996.1), including bus signal timing, central processing unit (cpu)
family, direct memory access (DMA), interrupts, serial ports, network interfaces,
mass storage, and standardized software. Both 8-bit and 16-bit bus versions are
available.
PC/104 technology allows engineers and programmers to leverage their existing knowledge of PC-compatible hardware and software to develop embedded systems that meet the strict requirements of medical regulatory agencies. In some cases, systems can be developed in as little as three to six months. With hundreds of commercially available standard PC/104 products available, systems designers can embed PC architecture without having to use a bulky, less-reliable motherboard- or backplane-based approach.
In many cases, the affordability and ease of use of PC/104 technology has allowed engineers to avoid reinventing the wheel—largely replacing proprietary embedded PCs based on individual chips, which are extremely costly and time-consuming to develop. Deeply embedded applications now have access to the same standard software tools available to a PC: application program interfaces (APIs), compilers, debuggers, development tools, operating systems, and utilities. This reduces the cost of software, as well as the learning curve for programmers and engineers.
Module Options
Common mechanical and electrical specifications of PC/104 modules make them interchangeable with products from other PC/104 manufacturers, many of which are ISO 9001 certified. Many PC/104 manufacturers have extensive design and manufacturing capabilities.
Because PC/104 is considered the embedded repository of desktop
standards, newer consumer-market technologies, such as biometrics fingerprint
sensors, PowerPC reduced instruction set computer (RISC) processors, and 10/100Base-Tx
Ethernet switches (IEEE 802.3u), have followed suit in the PC/104 form factor.
A variety of open-market PC/104 products are widely available for embedded applications,
ranging from cpus to expansion modules (i.e., video controllers, digital input/output
boards, field bus and network interfaces, power supplies, and data-acquisition
modules). Given that many medical control systems do not require the high overhead
of a large operating system (e.g., Windows) or a sophisticated graphical user
interface (GUI), but rather incorporate simple button controls or a minimal
GUI, they are often powered by low-end PC/104 Intel-based cpus (166–266
MHz). Scalable PC/104 cpu cards are available that give original equipment manufacturers
great flexibility for enhancing system performance to meet future needs.
These types of scalable cpu modules have a removable cpu core
(similar in size to a Personal Computer Memory Card International Association
[PCMCIA] card) that can be replaced and upgraded by simply plugging a new one
into an onboard socket—with no soldering required.
PC/104-Plus version products expand the potential use for PC/104 into even broader,
performance-intensive applications, such as high-speed graphics and data processing.
This type of PC/104 board implements both a 32-bit peripheral component interconnect
(PCI) local bus and the standard 16-bit PC/104 (ISA) bus to support high-performance
Pentium and PCI peripherals.
As more medical devices incorporate Internet connectivity, PC/104 Web controller modules capable of controlling off-site electronic devices are becoming available. Ideally suited for rapid transmission control protocol/Internet protocol (TCP/IP) software development, these types of boards typically use an Ethernet connection to control devices over a local-area network (LAN) or wide-area network (WAN), enabling in-home care equipment to be remotely controlled and monitored by doctors and hospital staff.
Stackable Embedded Board
The PC/104 specification is exemplified by its rugged, reliable
pin-and-socket header that allows PC/104 modules to be stacked on top of each
other. These modules replace a PC motherboard's 36- and 62-position edgecard
connectors. The 104-pin stack-through bus connector in a PC/104 system eliminates
complexity, cost, and bulk. Motherboards and back planes are no longer required.
Stacked PC/104 modules, which are spaced 0.6 in. apart, are traditionally attached to each other through four corner mounting holes using aluminum or nylon threaded standoff spacers. The sturdy mechanical connection of a PC/104 system provides much greater shock and vibration tolerance than a desktop PC. This connection also enables simple removal of cards for maintenance or upgrades. This inherent rigidity is especially important for portable or mobile medical devices, because they can be moved during storage, handling, and operation.
A bus drive in a standard desktop PC is 24 mA. PC/104 has a more relaxed drive at 4 mA, which reduces power consumption to typically 1–2 W. This reduced power minimizes heat buildup and reduces electromagnetic interference (EMI). Despite its modest bus drive, a PC/104 stack can typically support up to eight interconnected modules in a single embedded system. When more than eight modules are placed in a stack, ac termination boards can be used to increase data integrity and system reliability.
Typical Configurations
PC/104 technology is extremely flexible in how it can be used in medical applications. A vertically stacked configuration is common and provides inherent structural rigidity and small mass, enabling its use in confining or tight spaces. Alternatively, cards can be placed as a mezzanine expansion module on top of a baseboard or other standardized form factor, such as EBX (Embedded Board, eXpandable).
Some baseboards, such as the PC/104 Quad Motherboard, are designed with four PC/104 headers to allow PC/104 modules to be distributed side-by-side for a flatter and thinner system profile. Others integrate a power supply, medical instrumentation bus, and application-specific interfaces.
PC/104 modules can also be mounted directly to septum plates within enclosures or secured within a railed card cage, which uses four aluminum rails and two end caps bolted together to support the PC/104 card stack. This kind of mounting solution is commonly available in lengths of 4–10 in., accommodating up to 14 PC/104 cards.The mounting rails have precision-machined slots to secure the four corners of each circuit board, which eliminates the need for tedious placement of spacers and standoffs between circuit boards.
These slots enable quick removal and replacement
of defective modules or reorganization of the stack as it is being configured
for thermal management, connector orientation, and cable routing. With cards installed,
an entire card-cage assembly can measure as small as 4 in. on a side.
Recent innovations in PC/104 board mounting components have expanded mounting options to also include incremental card cages sized to the exact number of PC/104 boards in the card stack. Made of orthogonal polypropylene, this snap-together card cage provides four solid points of attachment for every card in the stack.
And, like the railed card cage, this type of construction makes the overall assembly more robust and shock resistant than a card stack bolted to a baseboard. Both incremental and railed card cages are improvements over a standard ISA bus configuration with card guides and only a single mounting bracket on the end of each card.
Mounting for Shock and Vibration
Movements, especially in mobile equipment, can often subject systems to mechanical stress that eventually degrades performance. PC/104-based systems have proven their ability to handle shock and vibration in some of the harshest environmental conditions, including on military combat aircraft, space shuttle payload bays, oil drilling ships, and overhead industrial cranes. Typical mean time before failure for PC/104 modules at 55°C is approximately 150,000 to 650,000 hours in a fixed application and 30,000 to 70,000 in a mobile application.
Although a PC/104's 104-pin header inherently helps to maintain electrical connectivity during vibration, board connectors can optionally be locked down to achieve superior reliability. Furthermore, systems using PC/104 boards mounted in a railed card cage can attach shock isolators of 70-durometer Silastic material to the outsid perimeter of the cage to isolate all points of contact between the rails and an enclosure.
These silicone rubber isolators help to absorb potentially damaging vibration energy and reduce G-loads on three axes by up to a factor of 10.
When mounting a printed circuit board (PCB) or a railed card cage to a septum plate, threaded Silastic standoffs can be used in lieu of traditional nylon or metal spacers to suppress shock and vibration. These shock-absorbing standoffs measure 0.75 in. long and 0.5 in. in diameter and are available in either 4-40 or 6-32 threaded versions.
Thermal Management
Although most medical devices are primarily used indoors, it
is important to note that PC/104 cards support a wide range of operational temperatures—
0° to 70°C in standard modules and –40° to 85°C in extended-temperature-rated
versions. Complementing the PC/104's inherently robust design are a variety
of intelligent thermal-management and power modules, as well as board-mounting
options that enhance the ability of medical equipment to handle harsh thermal
conditions.
Because most temperature issues stem largely from a system's own power supply, temperatures can be regulated using a variety of PC/104 heat sinks and fans to maximize airflow and conductive and convective cooling. High-profile, finned heat sinks designed with an integrated fan and 104-pin PCB connection can be stacked on top of dc-dc convertors within an enclosure to offer significant internal heat-dissipation abilities. Alternatively, when heat transfer by airflow is not practical or sufficient, power supplies can be given a direct thermal connection to the body of the outer enclosure through low-profile conductive aluminum heat sinks. Externally mounted heat sinks—the length of the extrusion sides or enclosure end caps—can further dissipate heat away from the system.
Some of the most widely used radiotherapy devices rely on embedded PC/104 computers with external fans. One particular series of medical accelerators incorporates a liquid-crystal display control unit that deals with heat generation through a 12-V fan mounted on the exterior of its aluminum enclosure. The fan efficiently cools the PC/104 stack, which runs on an Intel-based x86 processor.
PC/104 fan cards are a cooling option for embedded boards within a sealed enclosure. These cards create an effective push-pull circular cooling pattern using dual 5-V fans mounted on a PC/104 form factor. The fans maintain an even temperature inside the sealed enclosure, eliminating failure-causing hot spots, as well as providing heat transfer to the outer enclosure. Some fans are available with thermostats that don't require processor interruption. These environmental fan cards can monitor temperature intelligently, based on multiple digital inputs, outputs, and connections to external temperature sensors.
Moreover, system developers can opt for a high-efficiency power supply, such as a 90-W, 90%-efficient dc-dc convertor. Power supplies offering greater efficiency, local temperature monitoring, and intelligent shutdown capabilities often do not require a heat sink or fan in many applications.
PC/104 Enclosure Systems
Most medical electronics applications need protection from humidity,
moisture, dust, and EMI. Without EMI shielding, electronic devices could interfere
with each other and malfunction. Humidity could cause connections to corrode
and short. Dust could interfere with fans or moving components. The reliability
of critical-care devices could be seriously compromised if such factors are
not considered. In most cases, an effective en closure system guarding a PC/104
computer can successfully address these concerns.
supply, generates less heat for PC/104 systems.
Commercial enclosure standards published by the National Electrical Manufacturers Association (NEMA; Rosslyn, VA) are a useful gauge of enclosure ratings. The NEMA rating system ranges from Type 1– to Type 13–rated canisters, offering indoor and outdoor protection from various environmental conditions, depending on the rating. Typical PC/104 enclosure systems provide a Type 4 or 4x rating, meaning they offer protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, hose-directed water, and corrosion. They are also undamaged by external formation of ice.
Cases are most commonly made of aluminum, plastic, or steel materials. Plastic and aluminum do not rust; however, steel is stronger, and it can be coated with an anticorrosive material to prevent oxidation. Choosing the right enclosure for a particular medical system ultimately depends on the environment in which the device will be used, as well as variances in size, weight, and functionality.
Common aluminum enclosures, which are between 0.06 and 0.19 in. thick, come in various dimensions capable of housing up to 14 stacked PC/104 boards. Chassis are primarily made of aluminum alloy 6061 (T6) to maintain durability and minimize weight (beginning at less than 1 lb). Some aluminum enclosures have 0.10-in.-spaced internal grooves along two opposite inside walls to increase surface area for heat transfer. The grooves provide a mounting mechanism for septum plates, upon which the individual PC/104 cards or railed card cages can be placed. The cards easily slide in and out, as required, for maintenance.
For most enclosures, end-cap treatments largely determine the NEMA rating of the enclosure. With the ends of each extrusion chassis drilled at all corners, thin dust caps or NEMA-rated end caps can be bolted on. Conductive EMI gaskets can be inserted into the NEMA caps' 0.375-in. grooves to help maintain seal integrity and create a full ground connection, protecting the system from external voltage. A conformal coating further resists humidity on PC/104 boards mounted inside.
Maintenance and Upgrades
The modularity of a PC/104's design and its related mounting
accessories can help lengthen the product life of a medical device and simplify
field maintenance. Life-cycle issues are also addressed by the inherent backward
compatibility of the PC architecture (e.g., Pentium cpus with Windows 98 still
run disk operating system [DOS] applications).
Embedded systems using these building blocks can be upgraded or repaired without a major redesign by simply adding or swapping out PC/104 boards in a stack. This modularity facilitates the addition of new capabilities as requirements or technology change. Even custom baseboards or motherboards can leverage PC/104's modularity.
Future system requirements can be anticipated by incorporating a PC/104 mezzanine space onto a baseboard that can be used to add future PC/104 expansion cards. Adding mezzanine space early in the design frees such boards from technology obsolescence and ultimately speeds upgraded designs to market.
Card cages, removable maintenance access panels, and enclosure end caps with PC connector holes also enable quick access to the card stack and connectors when upgrades to software or hardware are required. When accessing a particular board held in a card cage, for example, only four bolts must be removed to gain full access to the board stack. This is in contrast to the time-consuming disassembly of a system held together with traditional standoffs between each board.
Bus and basic input-ouput system (BIOS) diagnostics can be done efficiently by temporarily adding a variety of standard PC/104 modules specifically designed to troubleshoot and debug embedded systems. Some of these diagnostic boards are especially useful as an alternative to an ohmmeter during environmental stress testing and other controlled tests to identify bus line and system failure points.
Conclusion
PC/104 technology dramatically improves time to market for embedded-systems development because it leverages the same readily available development tools used with personal desktop computers. Its compact size and rugged packaging represent a viable solution for reducing size and providing durability.
Moreover, PC/104 is a widely accepted open standard, which enables medical device manufacturers to configure an electronic system using interchangeable modules from vendors worldwide that supply hardware, software, and engineering services to support the growing PC/104 and PC/104-Plus standards.
PC/104 Resources
E-Zine of PC/104 Control Systems, 415-903-8304, http://www.controlled.com/pc104
PC/104 and PC/104-Plus Specifications; Embedded PC/104 Consortium (association), 650-903-8304, http://www.pc104.org
PC/104 Embedded Solutions (magazine), 810-774-8180, http://www.pc104-embedded-solns.com
Mike Southworth is responsible for corporate communications at parvus Corp. (Salt Lake City). He can be contacted at Mike_Southworth@parvus.com.
Copyright © 2002 Medical Electronics Manufacturing
