
TL/H/6750–33
FIGURE 30. A simple carrier amplitude detector with output low when carrier is detected
TL/H/6750–34
FIGURE 31. A simple linear analog audio transmitter and receiver are shown.
The carrier and 1.6V inputs are derived from the carrier detector ofFigure 30.
The remaining 2 LM339 comparators may be used to build the carrier detector circuit.
Communication and System
Protocols
(Continued)
sion in a system. Its first purpose is to put data in a base-
band digital form that is more easily recognized as a real
message at the receive end. Secondly, it incorporates en-
coding techniques to ensure that noise induced errors do
not easily occur; and when they do, they can always be
detected. Lastly, the software algorithms that are used on
the receive end to decode incoming data prevent the recep-
tion of noise induced ‘‘phantom’’ messages, and insure the
recovery of real messages from an incoming bit stream that
has been altered by noise.
2. System protocol: the manner in which messages are co-
ordinated between nodes in a system. Its first purpose is to
ensure message retransmission to correct errors (hand-
shake). Secondly it coordinates messages for maximum uti-
lization and efficiency on the network. Lastly, it ensures that
messages do not collide on the network. Common system
protocols include master-slave, carrier detect multiple ac-
cess, and token passing. Token passing and master slave
have been found to be the most useful since they are inher-
ently collision free.
Both protocols usually reside as software in a single micro-
controller that is connected to the LM1893/2893 I/O. In any
case, some sort of intelligence is needed to process incom-
ing and outgoing messages. UARTs have no usefulness in
18