JPS5779747A - Coupling circuit - Google Patents

Abstract

PURPOSE:To prevent the lowering in the impedance of a bus transmission line, by constituting a transmission drive circuit which is interrupted other than signal transmission. CONSTITUTION:A stationary base bias voltage is not applied to a base terminal of a transmission transistor (TR)312, which turns on only when the modulation signal is positive. Thus, at the reception state where no modulation signal is applied to the base terminal, this TR312 is non-conductive and it is not a load to a bus transmission line 1. If data are transmitted from a master station not shown at this reception state, this signal is received at a receiver 303 with high input impedance. With the constitution like this, even if a plurality of terminals are connected to the line 1, since the transmission TR is non-conductive and the input impedance of the receiver is high, the impedance of the line 1 is not decreased.