Robert,

While the '51 is not a drop in replacement for 8048/9 electrically (especially if the port expander is used)it was intended to represent an upward replacement.

The '51 has a 8048 timer compatibility mode (13 bit with prescaler) and a lot of mnemonic (source) compatibility for non architectural specific characteristics (i.e. the paging of code with it's limited JMP capability).

My first '51 source was just an edit of code previously written for 8049.

take a look at the 8048 application, you might find replacing it with a 8051 easier than you initially imagined.

regards,
p