I bought an ATMEGA board from Futurlec to use as a controller in my new robot I’m building, since it has the wonderful Atmel ATmega128 microcontroller and several 10-pin expansion connectors and RS-232 transceivers already on the board.
I was frustrated when I couldn’t get my brand new ATAVRISP mk II USB programmer to talk to the board (through a 6-pin to 10-pin adapter I made). I discovered that there was bus contention on the UART0 Receive Data signal because of the way the board is designed. The RS-232 transceiver chip on the board was always driving the PE0 pin, but this is a shared signal pin. Its other purpose is to be the Programming Data In (PDI) pin. My programmer couldn’t drive a low signal value on the pin because the RS-232 transceiver was always driving high. So I hacked the board to get rid of the bus contention and to allow my ATAVRISP mkII programmer to work with it.
Click HERE to view the PDF that shows the edits I made to prevent bus contention on RXD0 during programming.