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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

MAN MACHINE INTERFACE

Most instrumentation that doesnýt use a PC for displaying information uses an LCD. From the bottom-up perspective, LCDs are ubiquitous, inexpensive, and a natural choice for a display in our controller.

Itýs tough to know what type of display an application developer will require. If we choose one too small, the developer will have to design a larger display onto the hardware and write drivers. If we choose one too large, we burden the controller with unnecessary cost.

The hand-held requirement for packaging creates certain limitations for us as well. LCD modules with integrated glass, PCB, and bezel are available but are rather bulky.

A half dozen LCD vendors now offer chip-on-glass (COG) displays as a standard product. The 128 ý 64 graphic COG module from Hantronix (PN HDG12864F-1) is less than $20 in quantities of 500. It measures 77 mm ý 51 mm ý 3 mm and should fit nicely into a hand-held enclosure.

If we design the drivers correctly, the application developer will be able to display a variety of font sizes and graphics. The API will let the developer display simple text or complex graphics. This should be a suitable display interface for most measurement or data-logging systems.

The keypad is more difficult to pin down. The technology chosen must enable the developer to customize the keypad at a relatively low cost. This could mean either changing the number and/or configuration of keys, or simply changing the legend. The keypad should be versatile enough to allow full alpha and numeric entry and have several "function" keys or softkeys.

Physically, the keypad should be sealed against dust and water. There must be a careful integration with the physical package.

I have contacted two companies (GM Nameplate and Miller Dial Corporation) about keypad options. A future article in this series will fully explain the keypad construction.

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