This is a homemade DS89C420  programmer that was built by Glenjoy ( global moderator of electronicslab philipppines forum ). The programmer uses the MAX233
RS232 interface to eliminate those external capacitors required by MAX232. Although a bit
expensive, it is advisable to use zero insertion force sockets on the
programmer to maximize the life of the IC pins. For schematics and pcb layout, check out this link http://www.electronicslab.ph/forum/index.php?topic=84.0