Serial port with PIC

Thread Starter


Joined Aug 11, 2005
Some time ago I built the circuit at (diagram at bottom of page). With the USB-to-serial adaptor on my laptop it did nothing, and with a real serial port, some data was being transferred but it was wrong.

Further research suggested that the accuracy of the internal oscillator is borderline for serial communication. So perhaps if I used a crystal it would work.

The other thing though is voltages. The page says,
"One of the other nice things about the program ... is that it can be used for direct connection to a PC (+5 volts=high, 0 volts=low) or by using a true RS-232 signaling connection (+5 to+15 volts=low,-5 to -15 volts= high) ... The schematic shown and the program listing are for a direct connection. If you wanted to use the standard RS-232 signaling protocol you would need to add a dual RS-232 transmitter/receiver chip like the Maxim 233 integrated circuit chip."
But I read more recently that a real PC serial port uses the larger +- voltages, and it's just the simulated USB ones that use 0-5V. (The "JDM" PIC programmer depends on those higher voltages for power, and won't work with a USB adaptor.) And all the other PIC serial port circuits I've found since (mostly using the UART on a larger PIC) include the Max232 or similar. So is the very simple circuit wrong? Does it even risk blowing something up?