RS232 Dev Board

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In the circuit above, the two 1N4148 diodes prevent any negative voltage getting into the rest of the circuit if either DTR or RTS would happen to go negative. The two transistors function as transistor switches. The transistors also act as inverters. We need to invert things since the pc serial port uses positive voltage to represent binary 0 (RS232 SPACE) and negative voltage (or lucky for us 0 voltage) for binary 1 (RS232 MARK). Of course the voltage regulator regulates the voltage with some decoupling capacitors. A zener diode can also be used to regulate the voltage. In my schematic, you can see that I developed this for my PIC projects, but other microcontrollers will probably have similar serial communications methods and similar supply voltage requirements.

One thing I needed to do to use this with my solderless breadboard was to connect the metal on the bottom of the breadboard to ground. You may need a similar sort of grounded shielding near your circuitry if you use this.

• NØXAS shows how to replace the BJTs in this circuit with FETs, which is simpler and (now that FETs have plummeted in price) cheaper.