Page 118 - Anatomy of a Robot

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COMPUTER HARDWARE 103
INPUT/OUTPUT (I/O)
No matter how good a processor is, it’s useless if it cannot communicate with the out-
side world. A computer can only process information as fast as the slowest link in its
communication chain. Besides traditional input/output (I/O), which we’ll get to in a
moment, other communication paths within a computer can slow it down. If the mem-
ory is too slow for the processor, everything slows down. When designing a robot sys-
tem the first time, be very careful to analyze the required communication bandwidths
throughout the computer circuitry.
Think of a person who is blind. A blind person can certainly think fast and figure
things out, but might take longer than most to assimilate visual or printed matter
through Braille. As such, a blind person might not be the best choice to be an air traf-
fic controller. I know I’m going to get into trouble for that statement, but it’s true. The
world is put together to suit sighted people and thus often puts blind people at a disad-
vantage. However, blind people might have advantages in situations where their spe-
cially trained hearing skills come to the fore.
So, too, certain processors have more I/O bandwidth than others. If the robot’s sys-
tem architecture calls for a processor to digest and process all the bytes coming in a
1000BT LAN interface at full speed, it’s a pretty good bet there won’t be any 8-bit
processors that can handle it.
We must evaluate many places inside the computer hardware to determine if enough
bandwidth will be able to handle the contemplated design. The list of considerations
includes the following computer components.

Buses

A bus is a communication path within the computer that carries data from one place
to another. Generally, a bus is a collection of wire traces on the PCB with a protocol
that defines the meaning of the signals on the traces. It is not possible to put more data
across the bus than the protocol claims it can handle. In fact, it is rare that the full,
raw bandwidth capability of the bus can ever be achieved. While planning the design
of the system, it is wise to derate the bus to 50 to 80 percent of the advertised band-
width. This is likely the fastest speed at which we will be able to drive bytes across
the bus.
Buses are generally designed by industry committees to solve particular data trans-
fer problems. Often, the bus will have been designed to enable multiple manufacturers
to build compatible equipment. Buses have characteristics such as width (analogous to
word length), bandwidth in bytes per second, voltages, and loading. Loading defines
the number of separate devices that can be connected to the bus at the same time.