Using a Dallas 89c430. Just like the 420, just a little more memory. The jumper, JP4, was set to Yes as to allow the device to be programmed.

I just tried it with it set to no, no change.

Craig, have you considered another PCB like the SBC, just more "bare bones?" The SBC is great for educational purposes, but alot of it is unnessecary in an application. A socketed 40 pin socket, an xram socket ant latch, and perhaps an eprom with some other minor external circuitry (clock, some pin headers for each port like on the SBC.) Alot of people building projects with 8051s would use these as they are cleaner than what can usually be made at home.