Page 95 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 6

ASCII has come to be called serial communications although virtually all communi-
cations are now serial. The reason for ASCII’s popularity was simply that these in-
terfaces were available for all microprocessors and were built directly into many
CPU chips.

The biggest disadvantage of ASCII is that it is limited to baud rates of 115 Kbps. The
proliferation of intelligent controls and the internet have brought into existence se-
eral new and better alternatives to serial communications. These networks have
both electrical and protocol definitions on how data is transmitted, but they do not
specify what the data means. The meaning of the data is determined by the applica-
tions protocol that rides on the network protocol.

USB and FireWire

Both USB (Universal Serial Bus) and FireWire (IEEE-1394) were designed to serve
as peripheral busses for PCs. They are therefore inherently master-slave in nature. The
USB interface is commonly used in low-to-medium bandwidth peripheral connec-
tions, ranging from image scanners to digital cameras. USB operates up to 1.25 Mbps.

FireWire is more expensive than USB, but much faster. Offering data rates up to 400
Mbps, it is most often found in computers that are designed specifically for high-end
multimedia editing.
Either of these busses can serve as a data acquisition bus on board a mobile robot, but
they are less suited for higher level communications.

CAN
The CAN (Controller Area Network) is a multi-master asynchronous protocol that
was developed for the automotive industry. Over the past decade it has come to
dominate inter-vehicle communications. In most modern automobiles, almost every
electrical object in the vehicle from the ignition to the automatic windows commu-
nicates by CAN network. There is wide support for interfacing to various instru-
ments and development software is available at a low cost. Manufacturers such as
Motorola and National Instruments offer chip sets and extensive support for CAN.
The presence of the CAN network has made possible diagnostics that have revolu-
tionized automotive repair and maintenance.

There are two electrical standards for CAN. The low speed (125 Kbps) standard
trades off bandwidth for the ability to continue operating if one of the two lines of

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