It sounds like you need to check carefully if the test program of the 80C517 is using extended features of the chip that are not available on the 89C55. Things to look at:

1) Siemens part has 2 serial ports. Are you using the 2nd port on the prototype?

2) The Siemens part has extended support of baud rate generation that the Atmel does not have. Is your test program using the dedicated baud rate generator of the 80C517.

It appears that you will also need to check the full program too. There are many features of the 'C517 that are not present on the 89C55.

Good Luck
Michael Karas