I have used Keil tools before with a TI micro and never had the problems I'm seeing now. We received a MCB251 board this week and swapped out the 80251 chip for an Atmel AT89C51RD2. On powerup no LED action. Installing a SIEMENS 80C32 or Phillips 87C51RA the LEDS do a sequence (similar to what happens when the 80251 is in). None of these processors can be programmed I just get download errors. I checked this with a couple of different computers and verified the RS-232 port I'm using now. Trying the 80251 eval software with the infernal Blinky program, no download to the 80251 chip. My understanding of this board is that the monitor is in the big EPROM, so no need to use a pre-programmed chip. When I have the 80251 chip installed and look at the serial output in a terminal program during reset I get a single "?" at 19.2 kb from the int DB9 with ext-uart sw OFF. But not with the other micros.
The C251 is NOT A C51. It has a different instruction set that is a super set of the C51. Therefore throwing a C51 based processor in will simply not work.
It has a 256K memory map (1 of 4 maps) and several other different features.
If you want use the ATMEL part try AT/TSC8x251G2D or the like. These parts are going to become scarce as Atmel is trying to discontinue them.
Stephen