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Bus Architecture
A hierarchy of considerations apply
as your design moves up the scale
I remember the first time I got involved in a bus retro-fit project.
That was about 20 years ago, at a company called ROLM.
ROLM is a manufacturer of telephone equipment for medium and large businesses.
At the time, the main backplane bus in the ROLM system ran at 4.5 MHz. This speed
limited the number of customers ROLM could serve from one product. If we could speed up the
bus, the product could serve more people.
My assignment was to figure out how to quadruple the bus capacity, boosting the operating
speed up to about 20 MHz. Looking back on it, an operating speed of 20 MHz doesn't sound
that difficult, but what you need to know to appreciate this problem is the length of the bus.
The bus was 75 feet long.
It snaked up and down through rack after rack of equipment, tying together literally
hundreds of PCB's. I came to recognize on this project that the difficulty of a bus
design is related not just to the operating speed, but to the relation between speed and bus length.
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