A Medical Electronics Manufacturing Fall 1997 Feature

EMBEDDED PCs

Rick Lehrbaum

Embedded computer-­compatible architecture can decrease development time and cost of medical products.

Today's medical equipment designers face a dazzling set of opportunities, both technological and logistical. Continual discoveries in basic science­including developments in such fields as physiology, biochemistry, and biophysics­are enabling the industry to create entirely new types of medical devices as well as to enhance existing ones. Equally significant is the potential offered by the rapid evolution of computer hardware and software technologies over the last decade. Powerful microprocessors, with vast program and storage memory capacities and supported by highly advanced software environments, allow dramatic increases in the sophistication of patient monitoring, process control, and data analysis. Functions that in the past required the resources of a mainframe or minicomputer can now be embedded directly within a medical instrument. Complex medical equipment also can be made user-friendly by virtue of highly intelligent graphical user interface software.

Medical equipment developers, however, also face an important challenge: the need to comply with a host of product safety requirements and other regulatory restrictions. Each new product must be skillfully and efficiently guided through a maze of complex and time-consuming FDA approvals. How can designers take maximum advantage of advances in technology, yet bring new products to market in a timely manner and within budget constraints? One solution is to cleverly combine product-specific proprietary hardware and software with off-the-shelf embedded computer hardware and software building blocks.

Embedded Computer Evolution

Embedded computer technology has come a long way since the birth of the microprocessor in the mid-1970s. There has been continuous exponential growth in both central processing unit (cpu) performance and memory capacity for program execution and data storage. In the 1980s, a few megahertz and a few kilobytes were the norm. By the 1990s, this became tens of megahertz and tens of megabytes. Currently, embedded computer cpu's are clocked at up to 200 MHz, RAM capacities are beginning to surpass 64 MByte, and mass storage is commonly measured in gigabytes.

As cpu speeds and memory capacities have grown ever larger, embedded computer applications have become increasingly decoupled from the underlying architecture of the embedded computer, and the real magic of the embedded system has become its unique software, interface technology, peripherals, and packaging. The result is that new product developers now spend more of their time being medical device designers rather than embedded computer architects. Increasingly, the embedded computer is perceived as a platform on which to run the application's software, and the preferred embedded computer architecture is that which optimizes the application's software development process, resulting in faster development cycles, reduced technical risks, and improved system sophistication. In today's fast-moving and competitive market, any and all such efficiencies are especially welcomed by the medical equipment industry, which is burdened by compliance and regulatory concerns.

An obvious way to increase the efficiency of embedded system development is to employ standardized, even off-the-shelf, hardware and software building blocks, if appropriate, thereby minimizing the need to design from scratch. How can this be accomplished? In response to this question, embedded system development teams have looked to the architecture of personal computers to provide a standardized hardware and software tool kit.

The Allure of PC Compatibility

The enormous popularity of the desktop PC has inspired a vast amount of software and hardware. If these products can be successfully adapted to medical systems, the medical electronics industry can benefit from the billions of dollars spent on those products' development. Use of a PC-compatible hardware/software architecture in embedded computer medical equipment applications can decrease development time and costs, reduce the cost of chips and peripherals, and minimize maintenance and support requirements. Among the key desktop PC technologies of interest to medical product designers are the following.

A baseboard with two side-by-side PC/104 sockets.

Central Processing Units. Driven by the high volumes of the desktop market, Intel architecture cpu's offer the best microprocessor price/performance ratios available. Because cpu vendors constantly vie with each other for faster and cheaper chips, speeds and features continually improve yet the prices stay roughly the same. Today's 64-bit Pentium processors cost little more than yesteryear's 16-bit 8086s.

Operating Systems. A wide range of desktop PC operating systems, including DOS, Windows 95, and Windows NT, as well as many real-time operating systems (RTOSs), support the PC architecture standard. Importantly, it's not necessary to port an RTOS to a system that complies with the PC standard, since most RTOSs already support the PC.

User Interfaces. Two areas of importance for product differentiation in the desktop PC field are ease of use and richness of display. Consequently, there have been great advances in graphical user interface (GUI) capabilities, both from the software and hardware perspective. Display devices, including CRT (cathode ray tube) monitors and flat-panel liquid crystal displays, for example, have improved in both resolution and color depth even as prices have decreased. GUI software is now a standard feature of such operating systems as Windows, and GUI acceleration hardware is a standard function of display controller chips.

Mass Storage. PC hard disk controllers have become obsolete, absorbed by the disk drive itself (IDE). An exponential growth in demand for hard disk capacity, itself fueled by exceedingly fast cpu's combined with increasingly complex operating systems and application software, has stimulated the R&D necessary to simultaneously shrink hard disk costs and explode capacities (now in the gigabytes). In addition, investments in solid-state storage technologies, needed for reliable laptop and notebook PC operation, have also yielded dividends in the form of cost-effective Flash memory technologies. Another important mass storage development from the desktop PC market is the CD-ROM, which offers extremely high data density and durability at very low cost, and connects directly to a PC's IDE interface without requiring an additional controller.

A PC/104 module.

Communications and Networking. As the speed, memory capacity, and functional sophistication of desktop PCs increase, ever larger quantities of data need to be communicated and shared. RS-232-C serial interfaces, the standard in the past, are no longer adequate. Fortunately, significant advances have been made in both wired and wireless communications technologies. Modem speeds are doubling annually and rates thought impossible a few years ago have now been attained. Ethernet, implemented within a single chip, is rapidly replacing RS-232-C as the standard computer-to-computer serial communications port. Plug-and-play software drivers that support Ethernet controller chips are included within Windows 95, Windows NT, and most RTOSs.

System Interfaces. A variety of new interfaces and peripheral devices have been created to support the requirements of desktop and laptop PCs. IDE, SCSI, and PCMCIA are all readily available and relatively mature, though all three continue evolving in capabilities and throughput. Newer options include the enhanced parallel port (IEEE 1284) as well as two new high-speed serial interfaces: Universal Serial Bus (USB) and FireWire (IEEE 1394).

Development Tools and Support. One of the greatest benefits of using PC architecture for an embedded computer is the rich assortment of tools and support available to system designers. All aspects of PC hardware, software, and applications are documented in books and magazines, and development tools for the PC platform are plentiful, cost-effective, and easy to use. Nearly every engineer, programmer, and technician is knowledgeable in the use of PC hardware and software.

The Medical Market's Unique Requirements

Clearly, PC technologies offer many exciting possibilities to medical equipment developers. However, embedded systems in general, and medical applications in particular, place severe demands and constraints on their internal electronics that are not applicable to desktop PCs. In the PC market, price pressures outweigh such concerns as product reliability, ruggedness, quality, and longevity. After all, the main objectives of desktop PC manufacturers are to minimize prices while continually incorporating the latest technologies in an effort to sell as many systems and system upgrades as possible. Embedded computers in medical equipment must satisfy a whole different set of objectives, including the following characteristics.

Figure 1. Mechanical dimensions of a 16-bit PC/104 module.

Figure 2. Stacking design of PC/104 modules.

Size and Weight. Size can be critical in systems destined for use in medical environments such as laboratories, emergency rooms, doctors' offices, and ambulances. Therefore, an embedded computer must take up no more space than the single-chip microcontrollers formerly used. Weight, too, is an important factor if the equipment is portable or intended for mobile use.

Power Consumption. System reliability is reduced by high heat buildup; therefore, it is important to minimize power consumption when replacing microcontrollers with embedded PCs. Power consumption and heat generation are especially important criteria in the design of portable and mobile systems.

Figure 3. A typical PC/104 mosule stack.

Shock and Vibration Resistance. Whether intended for fixed or mobile use, every medical product must be transported from where it is made to where it will be used. Desktop-PC motherboards and plug-in cards are notorious for needing adjustment by a trained technician after delivery, prior to use.

Such sensitivity to shock and vibration is not acceptable for embedded electronics, which are not expected to require service upon receipt. In the case of portable or mobile systems, system electronics will also undergo a wide range of gentle and harsh motions during storage, handling, and operation. Both large and small repeated movements may subject components, connections, and solder joints to continual mechanical stress until chips, modules, and boards become partially or fully dislodged or disconnected. In addition, connector pin conductivity can be degraded by corrosion resulting from electrochemical effects that are exacerbated by vibration. Ordinary disk drives may be inappropriate for program loading or data storage in these applications, so solid-state disks may need to be specified rather than ordinary disk drives.

Figure 4. PC/104 modules used as macrocomponents on an application baseboard.

Operating Temperature Range. Most medical systems are used in relatively benign indoor environments. Embedded electronics intended for such indoor use are typically rated for operation at temperatures up to 55°C. However, some medical equipment enclosures need to be fully sealed—to protect against spilled liquids such as blood or chemicals, for exampleשand air vents and cooling fans may not be permissible. In such cases, internal temperature can become elevated, which may require the embedded electronics to be rated for operation up to 70°C. In mobile or portable equipment, an extended operating temperature range of –20° to 80°C may be called for.

Power Supply Issues. Electrostatic discharge (ESD) and electromagnetic interference (EMI), both generated and received, are key concerns in medical applications. Highfrequency microprocessor clocks, which for PCs commonly fall in the range of 33–166 MHz, can easily interfere with low-level signal detection or stimulus generation. Also, medical systems often must operate in the presence of strong electromagnetic emissions from other devices situated nearby. Therefore, their embedded computer electronics must be designed with high noise immunity and low noise generation. Consideration also must be given to conductive radiation and susceptibility on power supply and I/O connections. Undesired system resets and data loss must, of course, be prevented, but the potential danger to humans from high levels of electric, electrostatic, or electromagnetic emissions is a far greater concern in medical applications and requires designers to incorporate preventive measures.

Quality and Reliability. Naturally, the required level of system quality and reliability depends on the particular application. Equipment used for noncritical-information data entry or retrieval, for example, can include fewer fail-safe mechanisms than systems performing life-critical patient monitoring or blood chemistry control. However, it is categorically safe to say that medical users are never as forgiving of system malfunctions or crashes as are the users of desktop PCs. Practically every PC user experiences messages such as "Fatal error #XYZ" from time to time, but such incidents are totally unacceptable in medical equipment, where consequences can range from loss of critical data to loss of life.

Product Life Span. Regarding product longevity, too, the priority of desktop PC users and manufacturers runs counter to that of the medical device industry. Desktop PC vendors strive to bring out new technologies constantly, and the typical half-life (to obsolescence) of PC chip sets is around three Comdexes (1 Comdex = 6 months). Clearly, while it may benefit PC manufacturers to sell their customers a new motherboard, video card, disk controller, or network controller every year or so, this situation is unacceptable for manufacturers of medical equipment. Because medical products typically are two or more years in development, often followed by several more years to gain FDA marketing approval, medical designs cannot be based on components with life spans as short as 18 to 24 months.

The PC/104 Alternative

Although there are many potential benefits of using a PC-compatible hardware/software architecture in an embedded system design, there is little incentive for manufacturers of desktop PCs to cater to the special requirements of medical equipment designers. Indeed, doing so could be suicidal in the cutthroat consumer market. So the question becomes, Is there anything else that can allow medical product designers to take advantage of PC technology?

Fortunately, the answer is yes. The PC/104 embedded computer modules standard was introduced by Ampro in 1992 specifically to provide a modular building-block method of incorporating PC hardware and software technologies into embedded systems. Available from many suppliers, today's PC/104 modules are suitable for a range of nondesktop applications, including fixed, portable, and mobile systems. Basically, PC/104 modules repackage desktop PC functions in a manner that satisfies the ruggedness, reliability, and size requirements of embedded systems. They offer full hardware and software compatibility with the desktop PC architecture, but in the form of compact (=3.6 x 3.8 in.) self-stacking modules (see Figures 1 and 2). Prior to the availability of PC/104, the options for embedding a PC architecture were to use a motherboard- or back plane–based approach, which created a bulky and unreliable system, or to create a custom embedded PC based on individual chips, which was costly and time-consuming. PC/104 modules provide a space-efficient middle ground for many embedded applications.

The differences between PC/104 and the PC standard are primarily mechanical; there are no software differences. Table I summarizes typical PC/104 specifications (with values varying depending on the specific module used), and the following list highlights several features of the modules.

• Miniature form-factor. Instead of the usual PC or PC/AT expansion card form-factor (12.5 x 4.8 in.), each PC/104 module measures only 3.550 x 3.775 in. There are two bus formats, for 8- and 16-bit modules. Unlike the 8- and 16-bit expansion cards of desktop PCs, both PC/104 versions are the same size, although the 8-bit version omits the P2/J2 bus connector.

   

• Self-stacking bus. To eliminate the complexity, cost, and bulk associated with conventional motherboards, back planes, and card cages, the PC/104 expansion bus is based on a self-stacking (stack-through) bus connector. Multiple modules stack directly with each other, as shown in Figure 3. Stacked modules are spaced 0.6 in. apart and are securely attached to each other by four standoffs.

   

• Pin-and-socket connectors. Rugged and reliable 64- and 40-position male/female header connectors substitute for the standard PC motherboard's 62- and 36-position (P1 and P2) edgecard connectors. The PC/104 bus connectors contain gold-plated pin-and-socket contacts on 0.1-in. centers.

   

• Bus signal function and pin assignment. All PC/104 bus signal functions are identical to their counterparts on the desktop-PC bus. Their assignments to the 104 locations of the PC/104 bus connectors are defined in the PC/104 specification.

   

• Bus drive. To minimize power consumption (to several watts per module) and chip count, the PC/104 bus drive was reduced to 4 mA from the 24 mA used in the IEEE desktop PC spec. This reduction also permits HCT logic and many VLSI chips to drive the bus directly, without additional buffer chips. An additional benefit is lower radiated electromagnetic emissions. Despite its modest bus drive, PC/104 can typically support up to eight interconnected modules in an embedded system.

Although not explicitly included in the above list, an additional advantage of PC/104 modules is that they are designed specifically for use as embedded components within OEM products. Therefore, suppliers of the modules focus on providing the quality, reliability, service, and support demanded by this customer base.

Figure 5. A PC/104 plus module that features a PCI interface.

Figure 6. An EBX form-factor standard module.

Typical PC/104 Applications

Although configuration and application possibilities are practically limitless, there are two basic ways PC/104 modules are used in medical systems: as module stacks and as macrocomponents on an application baseboard.

Module Stacks. PC/104 modules are often used like ultracompact back plane-bus boards, as illustrated in Figure 3, except that the modules stack directly together without needing the back planes and card cages of traditional bus-based solutions. Highly compact PC/104 stacks can thus be placed directly within a medical system's enclosure, in an otherwise empty space. In this manner, the equivalent of an entire PC can be embedded within a system that might previously have required an external, attached PC for its operation. PC/104 stack enclosures are available from a number of vendors for packaging PC/104-based subsystems in both fixed and mobile environments.

Macrocomponents on Application Baseboards. Despite the popular image of a stack of PC/104 modules that fit in the palm of your hand, most PC/104-based system designs aren't actually based on a stacked approach. Instead, the PC/104 modules are distributed horizontally, plugged into custom application baseboards like multichip macrocomponents. This approach is shown in Figure 4. Such baseboards usually contain all interfaces and logic that are not available on—or, for whatever reason, aren't desired on—PC/104 modules. In typical medical equipment applications, the baseboard might include power conversion or power supply components, signal conditioning or isolating logic, specialized interfaces such as a medical instrumentation bus, and real-world I/O interfaces and connectors. Devices on the baseboard need not interface with the PC/104 bus, but might be included there simply to eliminate additional electronic assemblies.

What size and shape should the application baseboard be? Generally, it takes the shape of the system, which may be square, rectangular, or even round. Often, the application baseboard provides multiple PC/104 stack locations, allowing modules to be distributed side by side to achieve a flatter or thinner system profile. Whatever shape is chosen, designers should provide a spare PC/104 module location, or an extra 0.6-in. vertical clearance above the top module, to accommodate system upgrades or additional modules for system test, debug, repair, or other unanticipated future requirements.

The Evolving PC/104 Standard

The PC/104 Consortium recently adopted a PCI-extended version of PC/104 called PC/104-Plus (see Figure 5). Developed by Ampro Computers, PC/104-Plus is designed to support Pentium processors and PCI bus throughput for high-performance applications, which may include high-speed graphics, networking, or data processing. PC/104-Plus preserves full backward compatibility with PC/104, including the ability to coexist within a stack with PC/104 modules. Its availability enhances the flexibility of PC/104 modules and allows them to be used in increasingly performance-intensive applications.

Another approach to structuring a compact, reliable, fully compatible, and cost-effective embedded PC is to incorporate all the essential ingredients of a PC-compatible system onto a highly integrated single-board computer (SBC). An SBC can eliminate some of the bulk, weight, and costs associated with multiple boards or modules. However, although an SBC may contain all of the normal PC-compatible function interfaces (for keyboard, speaker, serial, parallel, disk, and network), there will usually need to be a means for adding application-specific functions and interfaces to adapt the embedded PC to its intended purpose. Providing a PC/104 (or PC/104-Plus) expansion stack location on the SBC not only permits later installation of additional off-the-shelf PC/104 modules, it also can serve as a standardized interface between the SBC and whatever custom electronics the application requires.

A recent collaboration between Ampro and Motorola has resulted in publication of the industry's first multivendor embedded PC SBC standard. Derived from the Ampro Little Board form-factor (5.75 x 8.0 in.), the new standard is called EBX, which stands for Embedded Board, Expandable. It provides an SBC form-factor that is large enough to accommodate a high level of functional integration and cpu performance, yet is small enough to be deeply embedded within a wide variety of applications such as medical instruments. An important feature of EBX is its inclusion of an onboard PC/104-Plus expansion location, which facilitates adaptation of the EBX SBC to specific embedded applications. The basic features of the EBX form-factor standard are illustrated in Figure 6.

Benefits of using an EBX embedded PC include a reduction in the number of modules and elimination of the electrical and mechanical interface "glue" associated with using a large number of modules. Another benefit is the ability of onboard devices, such as memory or video, to take advantage of local bus data rates rather than being constrained by bus interface speeds. Cost can also be reduced because of the reduction in the number of board assemblies. In comparing EBX with PC/104, the EBX approach may be preferred when the features of the EBX SBC closely match the application's particular requirements; on the other hand, its benefits diminish when additional embedded PC functions must be added via stacked modules or when the EBX SBC contains a large number of excess functions.

Conclusion

There are many reasons to use the PC architecture as the hardware and software basis for medical systems. However, standard desktop PCs fall far short of meeting the space, power, quality, reliability, and longevity requirements of most medical electronics equipment. The compact, modular PC/104 standard was developed specifically to provide PC architecture compatibility for embedded applications while meeting these stringent requirements. Designers of medical systems can use PC/104 modules in a variety of ways, including creating simple module stacks and plugging the modules into application baseboards in a flexible component-like design. Recent extensions to PC/104, including the PCI-enhanced PC/104-Plus and the new EBX SBC form-factor standard, offer even greater design flexibility. In sum, these new technologies can provide a means to simplify the development of embedded computers for a wide range of medical systems, resulting in faster project completion, reduced development costs and risks, and improved system features and sophistication made possible by the designer's ability to focus on the application itself rather than on the embedded computer architecture.

Rick Lehrbaum is executive vice president, strategic development, for Ampro Computers, Inc. (San Jose).

Design Resources

The following publications and sources provide additional information about PC and PC/AT-compatible hardware, software, development tools, and peripheral devices that may be helpful in designing systems based on embedded PC technology.

Circuit Cellar Ink (hardware/software magazine);
860/875-2199, www.circuitcellar.com

"Designing with PC/104, a Tutorial" (white paper); Ampro Computers, Inc.,
408/360-0200, www.ampro.com

EBX specification; Ampro Computers, Inc.,
408/360-0200, www.ampro.com

Embedded Systems Programming (software magazine);
415/905-2200

"Everything You Always Wanted to Know about SSD" (white paper); Ampro Computers, Inc.,
408/360-0200, www.ampro.com

IEEE P996 draft specification; IEEE Publications,
908/981-1393

IEEE 1284 parallel port information;
www.fapo.com/ieee1284.htm

IEEE 1394 information; IEEE 1394 Trade Association,
512/305-0200

PCI local bus specification; PCI Special Interest Group,
503/693-6232

ISA and EISA Theory and Operation; Annabooks,
619/673-0870, www.annabooks.com

ISA System Architecture; Mindshare, Inc.,
mindshare@interserve.com

PC/104 and PC/104-Plus specifications and PC/104 Resource Guide; PC/104 Consortium,
415/903-8304, www.controlled.com/pc104

PC/104 Embedded Solutions (magazine);
810/774-8180, www.pc104-embedded-solns.com