Communications bridge for impedance matching of a plurality of lines

Claim 1

A multiport bidirectional coupling circuit for causing the coupling through transmission lines of a central unit to a plurality of remote units in a polling type data communication system comprising: a pair of receive input terminals and a plurality of pairs of receive output terminals, each pair of said terminals being capable of being coupled to a transmission line having a characteristic impedance associated therewith; a receive input transformer means including a primary winding, a secondary winding and first impedance means, said primary winding being coupled to said pair of receive input terminals, said secondary winding being coupled to said first impedance means, said first impedance means being of such value that the effective input impedance of said primary winding is substantially the same as the characteristic impedance of said transmission line capable of being coupled to said receive input terminals; a plurality of receive output transformer means, each including a primary winding, a secondary winding, second impedance means and the primary winding of each receive output transformer means being coupled through said second impedance means and said isolation means included therewith to said secondary winding of said receive input transformer means, and the secondary winding of each receive output transformer means being coupled to one pair of receive output terminals, the value of each of said second impedance means being such that the effective impedance of said secondary windings of each receive output transformer means is substantially the same as the characteristic impedance of the transmission line capable of being coupled to said pairs of receive output terminals; a pair of Transmit output terminals and a plurality of pairs of transmit input terminals, each pair of said terminals being capable of being coupled to a transmission line which has a characteristic impedance associated therewith; a transmit output transformer means including a primary winding, a secondary winding, and third impedance means, the primary winding of said transmit output transformer means being coupled to said third impedance means, the secondary winding of said transmit output transformer means being coupled to said pair of transmit output terminals, said third impedance means being of such value that the effective input impedance of the secondary winding of said transmit output transformer means is substantially the same as the characteristic impedance of the transmission line capable of being coupled to said pair of transmit output terminals; and a plurality of transmit input transformer means, each including a primary winding, a secondary winding, and fourth impedance means, the primary winding of each of said transmit input transformer means being coupled to one of said pairs of transmit input terminals the secondary winding of each of said transmit input transformer means being coupled to said fourth impedance means such that said fourth impedance means furnishes isolation between the transmit input transformer means within which it is included and the other transmit input transformer means, said coupling circuit further including means for coupling said third and fourth impedance means, the value of said fourth impedance means being such that the effective input impedance of said primary winding included within that transmit input transformer means is substantially the same as the characteristic impedance of the transmission line capable of being coupled thereto; said fourth impedance means including first and second resistors and a potentiometer, said potentiometer having two end terminals and a wiper arm; said first resistor being coupled in parallel with said secondary winding, the end terminals of said potentiometer being coupled in parallel with said first resistor, and said wiper arm of said potentiometer being coupled to one end of said second resistor, said means for coupling said third and fourth impedance means connecting the other end of said second resistor through said third impedance means to the secondary winding of said transmit output transformer means, the values of said first resistor, said second resistor and the resistance of said potentiometer between said end terminals thereof being such that said second resistor acts as an isolation resistance and said effective input impedance of said primary winding of each transmit input transformer is dependent upon the combination of said first resistor and said potentiometer.