I have seen at least one system that had a very similar problem. The 8051 on the embedded system had a crystal frequency that favored an RF modulation scheme that the processor was involved with but was not at all compatible with getting standard UART bit rates out of the 8051 serial port. On the PC side of that system there was a specially supplied COM board. (It was an off the shelf PCI based single port COMM board). Normally these are like less than $20. The board was custom modified with one of those field configurable oscillator modules to match up with the 8051,

Michael Karas