RS 232 interface - RS 232 interface without a negative supply voltage

7-76 - elektor july/august 1982 RS 232 interface A microcomputer is usually connected to a peripheral device, such as terminal, printer or teleprinter by using an RS 232 interface. This normally requires a positive voltage between +5 V and +15 V (logic "0") and a negative voltage of -12 V to -5 V for logic "1". The positive supply for the RS 232 interface can easily be derived from the unstabilised 5 V voltage of the computer. However, very often the negative supply voltage cannot be obtained from the computer because modern EPROMs and dynamic RAMs do not require a negative supply. If the device to be connected (for example a printer) is already equipped with an RS 232 interface, then a negative supply can be found at pin 3 of the RS232 connector in the standby mode. Capacitor C1 charges via diode D1 and supplies the transmitter (T1) with a negative voltage. T2 converts the negative level of RS 232 interface without a negative supply voltage the RS232 transmission into a CPU...