Piggyback Computing
A modular approach to building rugged computers carries many benefits, whether they're for a fixed installation, a portable application or even a wearable system. Modular systems based on mainstream, ruggedized desktop computer technology can make the grade for even the most demanding operating environments. And separating the LCD display— one of the most expensive parts of a compact rugged computer and among the most difficult to ruggedize—into its own modular interface is one way to give the best lifecycle logistics and value to the military.
Computer systems based upon specialized technology for narrow niches, in contrast, could become obsolete within a year. When the dust settles and the technology disappears, the systems won't be supportable anymore, and it'll be time to start again from scratch.
An open-architecture bus is essential for modular expandability in a rugged computer, and expandability is essential in real-world systems. To take a box called a computer, no matter how big or small, and say it's solely devoted to one function—communications, for example—is a waste of resources. There's no end of anecdotal evidence of users who have discovered, too late, that there are useful functions they'd like to add to their single-function box.
With ruggedized commercial cards, it's extremely easy to add features. Once again, the state of the art in desktop computers is the most supportable choice, the most economical choice and the best logistical choice.It's possible to ruggedize commercial cards and slightly modify their mechanical interface, though it isn't viable to just plug in a standard edge card PCI board in harsh environments. Some modification is required to the mating, but as far as the electrical interface, it's identical. In this context, many COTS companies like Argon have made use of ISA and several varieties of PCI including CompactPCl and PC/104.
Modularizing the LCD Display
The modularity of a rugged computer can be further enhanced by physically separating the computer from its display and configuring the system as two snap-together modules (Figure 1, A and B). As far as ruggedness is concerned, the same levels can be achieved Figure IA With separate computer and display modules, the computer module can be designed to mate with many different size LCD displays. by either an integrated one-piece design or a modular piggyback two-piece design. Argon manufactures both types and offers computers in both PC and Unix flavors with LCD displays from 6.4" to 22" measured diagonally.
The trade-off of the modular approach is a small amount of additional depth—approximately 1/4" in a typical system—but it has the edge in thermal management. And the modular approach is also a bit more costly due to additional sheet metal and some mechanical parts, but it provides additional flexibility, better upgradabilty and the best logistics of all.
Heat dissipation is far easier to manage in two boxes because each box takes care of its own heat. The computer and display modules each have totally independent heat sinking, which spreads the thermal load. Also, dual modules provide more contact with the air so much more can be done with natural convection that minimizes the need for fancier cooling mechanisms such as heat pipes, heat sinks or heat exchangers.
Shock and vibration are largely issues for the display, and most all-in-one rugged computer systems use LCD displays because of space and weight reasons. A single-piece integrated system will be bulkier and likely to break more easily, and the display is breakable enough because of the glass involved. In contrast, the display of a piggyback system will be thinner, lighter and easier to handle. Consequently, the piggyback design makes it easier to control its design tolerance to shock and vibration. The piggyback approach also Figure lB The LCD and main computer modules shown bolted together. allows the display to be remotely located from the computer, a valuable capability in many space-constrained retrofit applications.
Piggyback Design Protects Against Obsolescence
But the main advantage of a piggyback design is lifecycle logistics. Hardly a week goes by without an announcement of the next step in computers or in displays, and PC boxes and Unix boxes get obsolete very, very fast. Separating the computer and display functions offers customers the ability to take advantage of cost effective technology insertion. They don't have to throw away everything to make an upgrade or replace the whole thing, and the number of spares required is considerably reduced.
Extra care must be given, of course, to the physical interface between the computer and LCD display modules in a piggyback design in terms of easy mating and detaching, solidity, EMI shielding and other issues. It's a waste of resources today to field single-purpose computers when what are required are boxes that let users consolidate functions. The power of mainstream technology and modular design is that they give rugged computers flexibility, interchangeability and ways to expand and upgrade quickly and inexpensively.