Interface convertor

The circuit converts the voltage levels at an RS232 interface into current signals. In the quiescent state, send signal TXD, which has a level between -15 V and -5 V. is converted into a quiescent current of about 20 mA. In operation, the level becomes between +5 V and +15 V, depending on the set bit. At this level, the current loop must be interrupted. At the other end, in the quiescent state, the PLC sends a current of about 20 mA which is converted into a voltage between -15 V and -5 V.