Fsk modem for bidirectional loop communications system

Abstract

A modem which utilizes a method and means for coding a data signal for transmission and reception which includes the steps of generating a prefix signal which is a train of pulses at a multiple or submultiple of a selected baseband frequency, then generating a data stream following said prefix with the data stream having one binary state transmitted at said baseband frequency and the other transmitted at another frequency. The data stream is terminated with a suffix having a waveform different from that of the two binary states. A separate receiver is provided for each of the redundant highways. The receivers utilize a clock circuit synchronized to the data and a timer responsive to the clock for establishing the bit time. The incoming signal is modified by a factor so that a single decoding circuit can be used to decode the prefix as well as the data. A prefix counter counts the initial incoming pulses and upon receiving the number in a prefix indicates to the circuit that a carrier has been detected. A synch period which follows the prefix is detected as an additional indication that the prefix is finished and data is next. The first carrier received from the redundant highways is selected based upon completion of the prefix count and the detection of a synch period. Once the synch period is detected a bypass circuit is utilized to bypass the modifying circuit so that the incoming signals go directly to the decoding circuit without modification. A bad-bit circuit is provided to prevent passing on to the station any bad bits. Likewise a dead-line timing circuit is provided to reset the receivers after a predetermined time period has elapsed without a carrier being detected.