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MODULAR INSTRUMENTATION DESIGN

Part 1ýDefining the Project
by Bob Perrin

Start • The Problem ý The Solution ý Mechanical Interface ý Man Machine Interface ý Electrical Interface ý API ý In Closing ý Sources and PDF

ELECTRICAL INTERFACE

Trying to guess what I/O mix a user will need is tough. Coming up with a single controller that will be "everything to everyone" is impossible. A controller system that is "most things to instrumentation developers" is a lofty but achievable goal.

There are a finite number of sensor and actuator interfaces. Most I/O interfaces are just variations of a few basic themes. The real magic lies in the firmware that controls the physical I/O.

It is possible to build a controller that has configurable expansion ports for I/O. This will allow the developer to tailor the system to his application. If we are careful in the I/O module design, the tailoring will be seamless.

The idea of an expandable controller is nothing new. STD-32, S-100, PC104, VME, and PCI are just a few examples of expansion buses, but are obviously overkill for a simple hand-held controller. We will have to come up with a bus that is more suitable for a small system. Perhaps SPI, I²C, Microwire, or some other similar interface will be useful.

How do we squeeze an expansion bus into a hand-held controller? We could use 0.5-mm, 1- mm, or 0.050ý connectors. These connectors cost around $5 for a 2 ý 10 header and mate, so if the controller has just eight expansion ports, we would have $40 in connectors. This seems a bit extravagant.

Fortunately, there are other less expensive approaches. One that I have toyed with is building expansion modules that have a ball grid array (BGA) pattern on the bottom of small PCBs. In assembly, the expansion models could be placed on the main controller and reflowed with the rest of the controller.

Two problems exist with this approach. Inspection of BGAs is impossible, short of an x-ray machine. This can be managed if the assembly process is well controlled and QA electrical testing is adequate.

The second problem is physical space. If we use a BGA pattern to mount the expansion module to the main controller board, we lose half of our PCB area because no components can go on the bottom of the expansion module.

BGA patterns may be a good method for mounting expansion modules in more specialized systems, but I wonýt use this scheme in this project. I present it only as food for thought.

To allow application developers to tailor the feature set of the controller while maintaining a physically small board, we will provide a menu of options for the developer, who can select the exact I/O mix for the application. During prototyping, he can use a generic, but expensive controller, which allows expansion modules to be installed using 1.0-mm connectors. For production products, we simply integrate the items the developer selected on a custom PCB.

The custom PCB can be laid out quickly, if the expansion I/O is selected from a standard library of prelaid-out modules. If the developer orders PCBs a hundred at a time (a reasonable quantity for a niche-market instrumentation company), the cost of the custom PCB will be less than $15.

Assembly of SMT boards is highly automated, and there are a wide variety of assembly houses to pick from. It may seem counterintuitive to spin a custom PCB for each customer and call it a "standard controller," so letýs explore this a little further.

Many embedded-controller companies already tweak certain features for customers. For example, embedded-controller companies routinely install different resistor values in their products to configure analog channels. The PCB is just another board-level component.

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