To have the programmer talk to the host computer, and ultimately, the user, I chose to use an RS232 interface. I used one of the great little chips from Maxim. The one I chose to use was the MAX220, the lower power version of their simple RS232/CMOS converters. Why the low power version? I had one lying around. This is the Parts On Hand Programmer, you know.

If you would like to use the MAX232, you should read the data book, since with that part you can use smaller value capacitors.

Another thing to watch out for: To generate the valid RS232 signals, this chip doubles voltage, so the capacitors hanging off the chip will actually see greater than the voltage supplied to the chip. Again, check the Maxim data sheet. It is available free from their website, and they are also very nice about sending out samples.

You'll see that I connected a resistor between pins 11 and 12. This is my "poor man's" echo. You have the PIC listen on the pin 11 side, and change from input to output when you want to send data. This way you only use 1 I/O pin on the pic for serial. It makes it not true full duplex, but I thought it was a rather elegant solution.

I'm also fiddling with the CTS pin to give some feedback to the host computer when the programmer is busy doing something other than listening to the serial port. The programmer's serial port is interrupt driven, but I disable interrupts in certain timing critical subroutines.