line 15 for send transmission to 34 1 34 once the send permission signal is set to 34 1 34 by flip flop 6 6 the signal remains 34 1 34 until the send request signal becomes 34 0 34 or a collision occurs as will be described below even if data that has been transmitted by the transceiver 6 and reflected back by the reflector 4 is received by the photodiode 6 2 and the output of the data sense circuit 6 4 becomes 34 1 34 the output of the flip flop 6 6 sets the send permission signal to 34 1 34 and permits the luminescent diode 6 1 to emit light the and gate 6 5 is kept closed and does not set the send permission signal to 34 1 34 if the output of the data sense circuit 6 4 is 34 1 34 even if the send request signal is 34 1 34 hence the device 10 does not transmit data and waits until the send permission signal becomes 34 1 34 while keeping the send request signal 34 1 34 thereafter when the output of the data sense circuit 6 4 becomes 34 0 34 the flip flop 6 6 is set and the send request signal is set to 34 1 34 so that the device 10 starts transmitting the data the transmission data transmitted from the device 10 modulates the radiation of light from the luminescent diode 6 1 and sends light to the reflector 4 when device 10 finishes transmitting the data the send request signal is changed to 34 0 34 and the flip flop 6 6 is reset by an inverter 6 7 and an or gate 6 8 accordingly the send request signal is changed to 34.