Its likely either jtag or a UART (serial/console) port..
Are you familiar with the Sigrok (
http://sigrok.org ) -compatible-cheap fx2 based logic analyzers?
They are insanely cheap, some are as little as $8 on ebay..
Look for "24 MHz logic analyzer"
Make sure you get some probes with it.. some come with a cable and mini grabbers, some come with several cables..dupont cables, etc.