Page 261 - Sensors and Control Systems in Manufacturing

Networking of Sensors and Contr ol Systems in Manufacturing
The token is a control word, and each MAP node can initiate 221
communication only if it passes the token. It is interesting to note
that MAP nodes that are not a part of the logic ring will never pos-
sess the token, but they may still respond to a token holder if a mes-
sage is addressed to them. Token management is handled at the
data link layer (DL)2 of the OSI model. This layer controls the fea-
tures of the token application with respect to how long a token can
be held, the sequence of addresses that is to take place, and the
amount of time allowed for retrying communications before failure
is assumed. If the logic ring is broken at some point—for example,
if one equipment unit is no longer able to operate—the other nodes
will wait a certain length of time and then will reform the token
passing scheme. They will do this by using an algorithm, through
which the token is awarded to the highest station address in conten-
tion. The rest of the stations on the ring are then determined by the
highest-address successors. This process is repeated until the token
ring is re-formed.
Physical layer (PhL)1 involves encoding and modulation of the
message so that the digital data are transferred into analog and digi-
tal communication signals. Each MAP application requires a modem
for this purpose. The modem takes the extended message that has
been developed at higher layers and modifies it so it can be used to
provide an electronic signal to the communications medium. The
medium itself provides the means for transferring the signal from
User 1 to User 2.
MAP continues to be an important protocol approach for sensors
and control systems in computer-integrated manufacturing. For very
large factories, the broadband option is available, and for smaller fac-
tories a carrier-band system is also available. A number of vendors
now produce the hardware and software necessary to establish a
MAP network. However, such networks typically are quite high in
cost and, because of the complexity of the protocol, can also be diffi-
cult to develop and maintain. Thus, MAP is only one of several solu-
tions available to planning and implementation teams.

4.11 Multiple-Ring Digital Communication
Network—AbNET
An optical-fiber digital communication network has been proposed
to support the data acquisition and control functions of electric
power distribution networks. The optical-fiber links would follow
the power distribution routes. Since the fiber can cross open power
switches, the communication network would include multiple inter-
connected loops with occasional spurs (Fig. 4.18). At each intersec-
tion, a node is needed. The nodes of the communication network