text: the crosspoints shown in fig1 comprise the two transmission terminals 109 and 110 , a first unidirectional transmission path comprising the darlington connected transistors 101 and 102 and the series connected diode 103 between the terminals 109 and 110 ; and a second unidirectional transmission path comprising the darlington connected transistors 104 and 105 and the series connected diode 106 . the control circuitry comprising the transistors 107 and 108 , the resistors 112 and 113 , the power terminal 114 , and the control terminal 111 serves to selectively supply current to the darlington pairs to place all four of the transistors 101 , 102 , 104 , and 105 in the conducting mode at the same time . current supplied to the base of the transistors 102 and 105 by transistors 108 and 107 , respectively , must be returned by the external circuitry connected to one or both of the transmission terminals 109 and 110 . control signals applied to the control terminal 111 , in cooperation with power applied to the power terminal 114 , serve to control the current source transistors 107 and 108 . when the control signal on the control terminal 111 is such that the transistors 107 and 108 are functioning as current sources , current derived from a supply connected to the power terminal 114 will flow through the transistors 107 and 108 to the base of the transistor 105 and the base of the transistor 102 , respectively . for proper operation of the crosspoint , the current supplied by the current source transistors 107 and 108 should be sufficient to saturate transistors 105 and 102 , respectively , for all values of current between terminals 109 and 110 occurring under normal operating conditions . under these conditions , transistors 102 , 101 , 105 , and 104 provide two low loss , reverse poled , unidirectional transmission paths between transmission terminals 109 and 110 . signals of one polarity applied between the transmission terminals 109 and 110 will be transmitted via the first unidirectional transmission path comprising the darlington connected pair of transistors 101 , 102 and the series diode 103 ; while signals of the opposite polarity applied between the terminals 109 and 110 will be transmitted via the darlington connected pair of transistors 104 , 105 and their series connected diode 106 . the diodes 103 and 106 of fig1 are utilized to protect the relatively low breakdown voltage emitter - base junctions of transistors 101 , 102 and 104 , 105 , respectively . the addition of the diodes enables the crosspoint in the off state to withstand higher voltages of either polarity between terminals 109 and 110 , than is afforded by the two darlington connected pairs of transistors . a second embodiment of a crosspoint in accordance with the present invention , which is shown in fig2 provides a breakdown potential between the transmission terminals 205 and 206 which is comparable to the breakdown potential of the circuit of fig1 without the use of separate diodes , between the transmission terminals 109 and 110 . in the circuit of fig2 two darlington connected pairs comprising the transistors 201 , 202 , 203 , and 204 are interconnected such that a base to collector junction of a transistor of one pair is connected in series in the transmission path of the other darlington pair . in fig2 one of the two unidirectional transmission paths between the transmission terminals 205 and 206 comprises the darlington connected pair of transistors 201 and 202 and the base to collector junction of the transistor 204 for one direction of transmission , and the darlington connected pair of transistors 203 and 204 and the base to collector junction of the transistor 202 for the other direction of transmission . the transistors 209 and 210 , the resistors 211 and 212 , the power terminal 208 , and the control terminal 207 are comparable to the control circuitry shown in fig1 . signals applied to the control terminal 207 in cooperation with a potential applied to the terminal 208 control transistors 209 and 210 to supply the current necessary to turn - on the transistors 202 and 204 , and consequently , the transistors 201 and 203 . when in the on state , the transistors 201 , 202 , 203 , and 204 provide a low impedance , bidirectional transmission arrangement between the terminals 205 and 206 . as in the circuit of fig1 the control currents for the transistors 201 through 204 are returned via external circuitry connected to the terminals 205 and 206 , and not shown herein . the resistors 121 , 122 , 123 , and 124 are connected across the base to emitter junctions of transistors 101 , 102 , 104 , and 105 , respectively , to provide a leakage current path around the transistors as a further breakdown protection device when the crosspoint is in the off state . similarly , the resistors 221 , 222 , and 223 are provided in the circuit arrangement of fig2 to provide a leakage path in the off state . the crosspoints of fig1 and 2 both provide low impedance , bidirectional transmission paths for communication signals of reasonable amplitude and are capable of passing dc control signals of both polarities . high level ringing current signals may experience some wave shape distortion of the current zero crossing when transmitted through such crosspoints . however , such distortion is not intolerable in the operation of a telephone system . fig3 shows a balanced transmission arrangement comprising tip and ring conductors labeled t and r respectively , a line scan circuit 301 for use in ascertaining the on - hook or off - hook state of the subscriber station 302 , two crosspoint circuits 303 and 304 connected , respectively , in the tip and ring conductors of the transmission path , and resistors 305 and 306 for returning the control current for the crosspoints 303 and 304 . either of the two embodiments of crosspoints shown in fig1 and 2 , may be utilized as the crosspoints 303 and 304 of fig3 . for example , if the embodiment of fig1 is used the terminals 109 and 110 are connected serially in the tip and ring conductors t and r , control signals are applied to the crosspoints at the terminal labeled c ( 111 in fig1 ), and a power source is applied to the terminal labeled p ( 114 in fig1 ). similarly , if the crosspoint embodiment shown in fig2 is used in the implementation of the arrangement of fig3 the terminals 205 and 206 are connected in the tip and ring conductors of fig3 . in that case , control terminal c is equivalent to terminal 207 and terminal p is equivalent to terminal 208 . the terminal p &# 39 ; between resistors 305 and 306 forms a connecting point for a second terminal of the power source to be connected to terminal p . it is to be understood that the above - described arrangement is merely an illustrative application of the principles of the invention and that various other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention .