PURPOSE: To attain the communication with the simplest bus transmission line even when a long range, by discriminating an ineffective data from an effective data in the data from a slave terminal at a master terminal, performing timing regeneration only at the effective data and fetching the data so as to eliminate the limit of the range between the master and the slave terminals.

CONSTITUTION: In Figure, 18, 19 are comparators, 20 is an OR circuit, V1 is a voltage slightly higher than zero volt and V2 is a voltage slightly lower than zero volt. When no receiving data exists, the circuit is in the non-current state with zero voltage. In monitoring a voltage across a resistor R with the comparators 18, 19 in figure, any output of the comparators 18, 19 is at "1" level with the presence of the receiving data, and the output of the comparators 18, 19 goes to "0" level at the non-current state without the receiving data, then the output of the OR circuit 20 keeps "1" level when the receiving data exists and "0" level while no receiving data exists. Thus, the presence of the receiving data is detected.