1. A communications system comprising: a master; a slave; a communication transmission means connecting said master and said slave, the communication means having a master channel, a slave channel and a call channel, said master channel, said slave channel and said call channel each being separate and logically distinct; said master having circuit means to selectively place said master channel in high and low logical states; said slave having circuit means to selectively place said slave channel to high and low logical states; said master and slave having protocol establishing means for enabling said master to control said call channel when said master channel and said slave channel are at a first logical relationship to one another, and for enabling said slave to control said call channel when said master channel and said slave channel are at a second logical relationship to one another.