System for controlling synchronization in a digital communication system

Abstract

A technique and implementation of generating and supplying synchronization and error checking signals to a serially transmitted data stream includes the generation of flag bytes which define the end boundaries of the serial data stream, an abort character for aborting the transmission of a frame of data in response to certain conditions, and a diagnostic evaluation character inserted into the data stream. In addition, the invention provides a technique for ensuring that the unique binary code by which a flag byte is defined occurs in the transmitted data stream only where intended. The flag code has been chosen to contain a prescribed number of consecutive one bits, (i.e. -- six) flanked by zeroes, and circuitry monitors the contents of a data frame as it is being serialized out for transmission to a remote terminal at times other than during flag transmission. When five consecutive one bits are detected, serializing out of the next bit in the data is interrupted, and a dummy zero bit is inserted prior to the next bit. As a result, the transmitted frame of data will contain no more than five consecutive one bits, except during the flag bytes, (or an abort character) thus ensuring proper synchronization of the end points of the frame. At the receiver terminal, detection and decoding circuitry also monitors the number of consecutive one's in the received data stream. When five consecutive one's are detected, the receiver decoder circuitry checks to see whether the next bit is a dummy zero bit. If the next bit is a zero bit, it is deleted so that the intended data will be correctly reassembled.