COM Express enables application-specific data acquisition systems

Story

October 12, 2011

Earle Foster

Sealevel Systems, Inc.

Military system designers often struggle with finding COTS technologies that can meet critical application requirements. Custom designs almost always offer the best opportunity to create an optimized design, but time to market and design costs can be prohibitive. A hybrid architecture using a Computer on Module (COM) for the core processing functionality with a custom carrier board for application-specific I/O can offer the best of both worlds. This approach eliminates the time and risk associated with a custom processor design while the less-complex carrier board provides the advantages of a purpose-built design.

When considering the architecture for a new military system, designers are faced with an important decision: use readily available COTS products or create or commission a custom design. Design costs and development time can be reduced significantly using standard products, and this approach has found recent favor as a way to lower development costs. However, finding readily available products to meet all application requirements, especially for I/O, can be problematic. Another major concern with COTS solutions is system longevity and design control during the 10+ year life cycle of most military systems. Costs for unexpected changes and product obsolescence can easily exceed the original savings realized from integrating a system using off-the-shelf parts.

On the other hand, a custom design can often meet the technical specifications and allow for tight design control, but might not provide the flexibility to adapt and respond to future application demands. Additionally, without a redesign, a custom system will not be able to take advantage of processor performance improvements and expanding memory options, thus limiting support for application program requirements or future operating systems. Effectively, the design will be frozen.

The good news is that this dilemma can be addressed in new product designs for applications requiring long-term availability and a large amount of I/O in a relatively small amount of space. The answer is a marriage of COTS and custom design using a COM Express module with a custom I/O carrier board. Using this architecture provides the freedom to exactly match the system I/O and mechanical requirements while providing an easy upgrade path for the core processing functions that are most likely to change, thereby extending the useful life cycle of the system.

Computers on Module speed designs, add versatility

Historically, designing custom single board computers that included application-specific I/O was a common solution for high-volume or extremely user-specific applications. Today, modern processors and advanced chipsets require designers with extensive experience and state-of-the-art equipment to implement. The Computer on Module concept eliminates the “hard part” of custom board design by combining the processing, memory, video, Ethernet, and USB functionality in a small, highly integrated module. These modules are not meant to operate stand-alone, but instead bring the processor bus and high-speed I/O out to an interface connector. The Computer on Module installs on a carrier board that provides application-specific I/O and external connectors.

COM Express, a widely supported implementation of Computer on Module design, is based around the latest high-speed serial bus technology and incorporates PCI Express, USB 2.0, Serial ATA, LVDS, and Serial DVO interfaces. The 2.5 GHz PCI Express 2.0 bus combined with today’s fast, low-power processors provide the bandwidth needed for a wide variety of applications including high-speed data acquisition and image processing.

COM Express modules are small and intended for rugged environments, simplifying integration into the mechanical form factor defined by the carrier board. Users can select from processor choices ranging from powerful Intel Core 2 Duo to low-power, low-heat Atom designs. Extended temperature range (-40 °C to +85 °C) models are available from many vendors.

Custom carrier boards for application-specific I/O

Meeting technical specifications with off-the-shelf products might not be possible or might result in unacceptable performance, size, environmental, or cost constraints. A custom carrier board can be “custom tailored” to meet all of the required system functionality outside of the core features supplied by the COM Express module. The carrier board can be designed to the mechanical footprint that best fits the application, and the bus signals necessary to interface the I/O are brought down from the COM Express board via the mating connectors. Since low-voltage differential signaling is used for the bus signals, COM Express offers designers a number of advantages over traditional parallel bus designs including improved noise immunity and fewer traces for easier signal routing.

Common I/O carrier board features include serial, analog, and digital I/O, all of which can be designed to exactly meet the I/O count, voltage ranges, and connector types for the signals to be interfaced. This flexibility is especially useful when connecting to legacy COTS or proprietary equipment that cannot easily be changed.

A company might do custom carrier design in-house if the necessary skillset exists and if they choose to invest the resources. A third-party carrier board provider can also offload the design and manufacture.

Optimized design meets challenging requirements

For a design such as the one mentioned earlier, a large number of I/O points, internal battery backup, and fanless operation over an extended temperature range could be implemented, all in a 1U enclosure. On first read, these specifications might seem unlikely to fit in the 17" (W) x 10.5" (D) x 1.75" (H) size required for the system in the target environment:

1.2 GHz fanless Core 2 Duo processor
Up to 8 GB RAM
64 GB SATA solid-state hard drive
Analog video
2 GbE, 4 RS-232/422/485, and 4 USB 2.0 ports
120 TTL digital inputs; 96 open collector outputs
32 single-ended, 12-bit analog inputs
Powered from 24 or 48 VDC with 5-minute internal battery backup
-30 °C to +60 °C operating temperature range

However, using a COM Express module that meets all of the core processing, memory, video, and disk interface requirements for this application can ensure success. The COM Express module also supplies the required USB functionality and one of the Ethernet ports. The remaining I/O and power circuitry – along with the connectors for the COM Express functions – are implemented on the custom carrier design.

For maximum flexibility, an FPGA is used on the carrier board to interface the processor buses to the I/O circuitry. Certain low-level functions to service the I/O are built into the FPGA using the device’s 17 Kb lookup tables and 700 Kb embedded block RAM to offload those tasks from the host processor. For example, the 32 analog inputs are continuously polled by the FPGA, and the values are retained in a table for easy access when needed by the application program. Future I/O requirements are also simplified because the FPGA can be easily modified to accommodate the changes. The architecture for the COM Express and custom carrier board is shown in Figure 1.

Figure 1: The COM Express module provides the core processing functions and mounts to an application-specific carrier board that adds the I/O, power, and all connectors.

(Click graphic to zoom by 1.9x)

Yet another advantage of a custom carrier board is demonstrated by the absence of cabling from the system. Implementing COTS technologies usually requires some type of cabling to locate the I/O connectors in the enclosure. The cable connections create potential failure points, especially in high-vibration, high-shock environments such as military vehicles. A custom carrier board can be designed to the exact mechanical dimensions to allow I/O connectors to be soldered directly to the PCB in such a way to allow external access, improving system ruggedness and MTBF. Figure 2 shows the target system including COM Express module, custom carrier board, and 1U rack-mount enclosure.

Figure 2: An example system shows a 19" 1U rack-mount computer with a COM Express module and custom carrier board.

(Click graphic to zoom by 1.9x)

Choosing the right approach for the application

The COM Express with custom carrier board architecture does not fit every application. Although using an off-the-shelf COM Express module speeds development time, a full COTS solution is still the fastest way to configure a system. But for many applications, the design flexibility, reliability improvements, and design control advantages make COM Express and a custom carrier board an excellent choice.

Earle Foster is Vice President of Sales and Marketing at Sealevel Systems, Inc. Earle has 25 years of experience in industrial computing and I/O connectivity solutions. He holds a B.S. degree in Computer Engineering from Clemson University. He can be contacted at [email protected].

Sealevel Systems, Inc. 864-843-4343 www.sealevel.com