Transparent error detection in half duplex modems

Claim 1

A modem for transmitting a plurality of streams of data bits over a communication channel in a half-duplex mode, said streams being separated by intermediate time periods T D , said modem comprising: encoding means for encoding said streams of data bits into corresponding streams of data communication signals; preamble means connected to said encoding means for generating preamble signals for transmission over said communication channel prior to the streams of data communication signals; timer means connected to said preamble means for detecting the time period T D between each successive stream of data bits and for disabling said preamble means if said T D is less than a preset threshold value; and parity word generating means having an input from a data terminal equipment module and generating a parity word corresponding to a previous transmission prior to the stream of data communication signals, said parity word being sent to a switch which controls data and said parity word transmission over said channel; and wherein said encoding means comprises a randomizer and a differential encoder for storing data further comprising clock signal generating means connected to said randomizer and said differential encoder which allow clocking signals to reach said randomizer and differential encoder and said parity word generating means when communication data signals are transmitted whereby the data in said randomizer and said differential encoder and in said parity word generating means remains unchanged while communication data signals are not transmitted.