Each time I wanted to hook up a micro-controller UART to my computer via serial port, I make an adapter with the following schematic:
For the capacitor, I use 47nF and for the diode, I use 1N4001. VCC is 5VDC regulated. The rest of the part values shown are self explanatory. Resistors are all 1/4 watt.
With this setup, If I deliver 5V to the TXDA net (aka output to PC), then the PC will recognize that bit as logic high. If however, I deliver only 3.3V then the PC doesn't want to recognize the bit.
The PC is able to output data to the micro through the RXDA pin just fine in all cases.
In my circuit, I connected pins 4, 1, and 6 of the serial port together and pins 7 and 8 together to make it compatible with all computers and to try to eliminate EMI.
Is there a way I can get this circuit so that it recognizes data at low voltages?
The reason I'm making such a circuit is because I'm simulating an HM-TRP wireless data module (that refuses to work at more than 3.9V) and I want to see and control the data on my computer so that later when I actually use the module instead of the computer, everything will then work.
And for those who ask, the voltage regulator I used to generate 3.3V from 5V is LM1117-3.3