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                                    T h ree
                           Cha p te r

                                                  FIBER
                                  FIBER OPTIC               FIBER OPTIC
                                  ENCODER         FIBER     ENCODER
                                  LINK            FIBER     LINK
                                  TRANSMITTER               RECEIVER

                                               NOISE IMMUNE
                                                3300 FEET
                                              5 Mbit/CHANNEL
                     FIGURE 3.33  Three-channel optical link.

                                 the basic building block for more complex links. Figure 3.33
                                 provides an example of a 5-Mbit three-channel link used to
                                 transmit encoder signals from a servomotor to a remote
                                 destination.


                          3.12.3 Digital Links—Carrier-Based
                          A carrier-based digital link is a system in which the frequency of the
                          optical carrier is varied by a technique known as frequency-shift keying
                          (FSK). Figure 3.34 illustrates the modulation concept; two frequencies
                          are employed to create the logic 0 and 1 states. This scheme is espe-
                          cially useful in systems where electrical “handshaking” (confirma-
                          tion of reception and acceptance) is employed. Presence of the optical
                          carrier is the equivalent of the handshake signal, with the data signal
                          presented by frequency.
                             Figure 3.35 illustrates a system where the logic of the fiber-optic
                          line driver recognizes the optical carrier to create a handshake
                          between terminal and processor.
                             Additionally, since the processor is capable of recognizing only
                          one terminal, the carrier is controlled to deny the handshake to all
                          other terminals once one terminal is actively on-line to the processor.


















                     FIGURE 3.34  Modulation concept.
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