Hardware Design
Hardware design was governed by an attempt to optimize four things: cost, ease of assembly, elegance, and stability (also known as "not-catching-on-fire-ness"). Following some research, it was determined that PCB board space has no marginal cost between 5 sq in and 85 sq in at prototype quantities, so space was no issue. Ease of assembly at first directed our attention away from surface mount components, but after assembling a simple kit using SMT devices, it was decided that SMD was faster and more elegant that through-hole for everything except ICs with lead pitch less than 50 mils. The largest SMD components available were chosen, with sockets used where possible. To achieve an attractive board, a two-layer board with soldermask was required. Silkscreening explanatory text onto the board was considered, but printed overlays were chosen as the LED labelling would be clearer with such a scheme. Stability concerns dictated a few practical considerations: ground traces had to be treelike, branching out from a single point to prevent loops and voltage drops, crystals and their associated capacitors had to be kept close to their ICs, and traces had to be wide enough to carry the necessary current.
The 14-segment LED display, showing the contents of the MAR.
A few design decisions merit explanation. The large 5x8 arrays of LEDs were chosen because they are well-suited to multiplexed driving, easy to solder, and cost-efficent compared to discrete components. The touchless keypad was chosen as it was cheaper, more elegant, and easier to deal with than an ordinary pushbutton matrix keypad. The dual 7-segment display and LCD combine to legibly display either memory contents or other text data. An RS-485 transceiver was included for future expansion over long distances; this protocol is simple, robust, and cheap, and is thus well-suited to this application.
The LCD displays the contents of ROM and RAM, and acts as a GUI when the device is receiving user input.
Alexandr Pshenichkin and Cortland Setlow
Swarthmore College