Medical Electronics Manufacturing Spring 1999
EMBEDDED SYSTEMS
Flexibility, Modularity, Density: Choosing Mezzanines for Medical Electronics
Pluggable mezzanine systems offer manufacturers the ability to design devices with embedded systems more quickly and cost-effectively.
Steve Weller
Embedded systems are customized to meet specific requirements for accuracy, precision, long-term stability, maintenance, various inputs and outputs, and measurements and computing power. Because of high costs, manufacturers want to avoid developing equipment that is actually too accurate or too precise or too fast. A system should meet the required specifications without exceeding them. That is where mezzanines come in. Pluggable systems enable manufacturers to combine the best of several components, whereas a single unit might contain all of the same pieces but in the wrong combination.
System designers can no longer afford the luxury of creating electronics from scratch for every program. The realities of cost issues and time pressures have forced designers to turn to standard products and interfaces and to the available products that support those standards. The success of a standard is based on how well it delivers practical solutions and on how many vendors support it with products.
A single-wide PMC module that combines 16 channels of analog input/output (I/O) and 16 channels of digital I/O is designed for use in industrial data-acquisition applications.
Mezzanines have succeeded in part because they provide a customized approach to embedded system design using commercial off-the-shelf (COTS) products. Mezzanines provide space and flexibility. Specifically, they maximize space in embedded applications and reconcile cost, reliability, and time-to-market issues for system designers. The primary purpose of these small daughter boards is to implement the most input/output (I/O) and processing functionality into the smallest space possible. The fundamental measure of any mezzanine standard is how well it addresses this need for functional density.
Mezzanine Modules: The I/O Supermarket
While mezzanines have been used on the virtual machine environment (VME) platform since the debut of the bus in the early 1980s, their adoption accelerated with the invention of the IndustryPack (IP) mezzanine module in 1989. Modularity allows system designers to provide the required functionality—and only the required functionality—and therefore to get a product to market more quickly.
PC·MIP is a high-performance, high-density mezzanine that uses the PCI bus interface.
Traditional bus systems encompass a motherboard or backplane with peripheral boards plugging in at right angles. Such configurations result in a bulky piece of equipment with a lot of expensive connectors. The mezzanine format is based on small boards that fit parallel to the motherboard, so that they consume much less space. The mezzanine module is still compatible with existing backplane technology because it simply piggybacks on top of it. It is also mechanically secure, unlike the right-angle approach, which introduces mounting problems and requires rails and a safeguard against vibration.
Certain characteristics distinguish IP modules from most other forms of daughter boards, including:
- Wide Application. IP modules have a simple interface, high data rate, bus independence, and comprehensive open specification.
- Small Module Size. The 45.72 x 99.06-mm dimensions allow significant slot (and therefore cost) savings. Four IP modules fit on a single 6U VME board. Although larger modules may save manufacturers from having to build two or three different versions of a board, larger modules do not deliver the same measure of flexibility or slot density.
- ID Space. The required identification programmable read-only memory (IDPROM) or electrically erasable programmable read-only memory (EEPROM) enables autoconfiguration as well as maintenance management.
- Keyed, Shrouded Connectors. The connectors are shrouded for pin protection and keyed for correct installation.
- Mechanical Integrity. IP modules can be secured to a carrier board with four stainless-steel screws and nuts, one at each corner. Such mechanical integrity is critical for high-shock or high-vibration environments.
Perhaps one of the best ways to describe the genesis of the IP mezzanine module is with a nonelectronics analogy. When you go to a grocery store, you buy separate ingredients. At home, you create a recipe based on a unique combination of those ingredients. Traditional VME boards are more like dining at a restaurant: you select a predefined configuration from the menu. It may fill a need; however, eating every meal at a restaurant would be too expensive and would offer minimal control over selection.
The IP mezzanine module provides clean, simple functions, such as serial, digital, analog, or avionics I/O. Modules include only enough electronics to implement a single well-understood function, such as RS-232 or CANbus. An engineer can then construct a system from these easy-to-use, cost-effective, off-the-shelf components.
A modular input/output IP-LCD.
Like the grocery stores that have evolved into supermarkets with large numbers of products on the shelves, IP modules offer a variety of functions and models. Manufacturers worldwide design and offer products to the same specification. The popularity and vendor base behind IP mezzanine modules fueled the approval of the specification by ANSI as a recognized open standard, "ANSI/VITA 4-1995, IP Modules."
PCI Migrates from the Desktop
The success of the mezzanine approach to system design extends to other configurations, such as the PCI mezzanine card (PMC), which takes advantage of the peripheral component interface (PCI). PCI will be used for virtually every local I/O bus on new microprocessor platforms and will be supported by all operating systems.
The fact that PCI would not fit on VME, CompactPCI, and other types of boards led to the formation of an IEEE standards working group in 1993. Several standards now designate the mezzanine cards. The parent standard, called "Common Mezzanine Card (CMC), IEEE P1386," defines the complete mechanics for both the mezzanine cards and the host's mechanical interface. Children standards are used to tie the mechanical specification to the local bus definitions. PMC is the first child standard (IEEE P1386.1), and it references the CMC standard for the mechanics and the PCI specifications from the PCI special interest group for the electrical, logical, and software layers.
The PMC mezzanine modules are larger (74 x 149 mm) than the IP counterparts. Frequently cited benefits of PMC (compared to VME) include lower cost, higher performance, a processor-independent local I/O bus, a software-independent I/O bus, and a large variety of I/O peripheral devices. PMC is a good replacement for a full-size 6U VME board, but it provides less density than IPs. Only two modules are accommodated on a 6U carrier. PMC offers two voltage options and numerous height, width, and connector options.
The connector locations on PMC modules support front-panel I/O more efficiently than rear-panel I/O. With the peak bandwidth of 132 Mbyte per second, typical applications for PMC involve communications and system expansion.
The Next-Generation PCI Mezzanine
In 1998, the emergence of a new specification, which is called PC·MIP, ushered in the next era of mezzanine modularity. With the IP standard approaching its 10-year anniversary, this new generation of mezzanine modules reflects the evolution of embedded system design. The baseline criterion required for development was that the mezzanine be based on PCI. The migration of PCI into embedded systems applications addresses the same needs as those in its desktop roots—high performance, a standard interface, ease of use, and low-cost, single-chip solutions for peripheral control subsystems.
With surface-mount connectors, the PC·MIP offers more space to mount components than on an IP module, despite its smaller size (49 x 90 mm for Type I or 49 x 99 mm for Type II). The space is more efficient because standard-thickness components can be affixed to either side of the module. Each module has a required, standardized injector/ejector, which protects the connectors and facilitates easy installation and removal.
PC·MIP is offered in two configurations: Type I for flexible I/O applications and Type II for front-panel I/O applications. The connector locations on PC·MIP modules are designed to provide an efficient electrical flow. The PCI interface is always located between the PCI connectors on one end of the board, and the I/O function is near the I/O connector on the opposite end of the board. The option at the I/O end of the board is to use either the flexible I/O connector or a front-panel I/O connector. If the flexible I/O connector is used, then routing to the rear-panel I/O connector is through the carrier board. In either design, the I/O traces do not interfere with the PCI traces, which tends to simplify the layout.
PC·MIP is continuing to gain momentum in the areas of interest and available products. Carriers are available for CompactPCI, PCI, and VME systems. PC·MIP modules offer functions such as memory, serial I/O, digital I/O, analog I/O, networking, bus interface, and video. The designers of the specification anticipate that PC·MIP modules will be useful to markets such as OEM, telecommunications, medical, and military because of their small, open-architecture, PCI-based mezzanine. Also, PC·MIP is suitable for the evolving standalone, single-board computer form factors, where density is at a premium. These are not the monster motherboards of yore. Moreover, they provide functionality unavailable with PC/104 and modularity unavailable with PMC. In a single-board, standalone design, expansion options are limited because there are no backplane slots, so six expansion slots are preferable to two.
While a few mezzanines have been discussed here, there are certainly others, such as PC/104, M-Modules, and SBus. Table I compares the specifications and applications of the mezzanines presented in this article.
| IP Module | PMC | PC·MIP | |
| Performance | 16 bit/8 MHz (16 Mbyte/sec) | 32 bit/33 MHz (132 Mbyte/sec) | 32 bit/33 MHz (132 Mbyte/sec) |
| Products available | Over 400 | Over 100 | About 10 |
| Size | 99 x 45 mm | 149 x 74 mm | 90 x 49 mm |
| Maximum in a 6U board | 4 | 2 | 6 |
| Interface | Proprietary synchronous bus | PCI plus "mode" signals | Pure PCI |
| Standard | ANSI/VITA-4 | IEEE 1386.1 | ANSI/VITA-29 (proposed) |
| Advantages | Small and modular Many products now available Simple memeory map address Proven long-term availability | Can carry complicated functions PCI components are redily avaiable High-performance mezzanine bus | Small and modular PCI components are readily avaiable High-performance mezzanine bus |
| Disadvantages | No integral front panel Low performance mezzanine bus | Big and not modular Complicated PCI memory addressing Not mechanically balanced | Complicated PCI memeory addressing |
Table I. Specifications and applications for IP, PMC, and PC·MIP modules.
The business-card size model for mezzanines seem poised for a long life in embedded systems design. The modules can be held in one hand easily, and a reasonable number fit on a traditional-sized host board. Although today it seems that a practical mezzanine cannot get much smaller, who knows what the engineers of tomorrow will deliver? As chips get more and more difficult to use, the make-versus-buy decision for designers moves steadily toward buy. Undoubtedly, I/O mezzanines will be viewed more as necessary components of embedded systems, offering the ability to upgrade and mix and match modules to meet I/O and performance requirements.
A Practical Implementation ExampleA developer of respiratory equipment chose to build its own central processing unit (cpu), primarily to contend with the strict regulations and indemnification issues of the industry. The concept of IP sites was appealing because the modules would facilitate efficiency in dealing with functions that tend to become obsolete frequently, such as Ethernet, SCSI, and specifically video (to drive overhead flat-panel displays). Such chips are subject to the nature of the consumer and commercial markets, which tend to follow a shorter life cycle than industrial or medical components. By incorporating these functions via IP modules, the device manufacturer would not have to respin the entire cpu card and (more importantly) reapply for FDA approval and other product certifications every time the function chip required replacement. In fact, the theory behind the decision was proved almost immediately. Almost simultaneously with delivery of the IP-LCD, the video chip manufacturer retired the chip. The IP supplier went back to the drawing board and subsequently delivered a new IP with the more current chip. This cycle has occurred three times, and in all cases the device manufacturer was provided with an IP that was consistent in form, fit, and function. The manufacturer was spared a more rigorous, expensive, and time-consuming integration process because only the IP required recertification. The rest of the cpu board was unaffected. In addition to protection against the pitfalls of obsolescence, using an IP eases system upgrades. When the manufacturer decided to switch video panels, only the IP required modification. Designing the video chip into the cpu card would have consumed engineering resources, extended the time to market, added costs, and complicated the certification process. |
Steve Weller is technical marketing manager for SBS Technologies, Modular I/O (Menlo Park, CA).
