Declan,

While the structure of your code is basically sound, you are going about the wrong way. Instead of reading from the port, which you are supposed to be doing, you are writing to it (ANL P2,A).

The idea is to get the low nibble, store it in 'Rx_Data', then get the high nibble and add it. "Getting" the nibble involves moving the port value into the ACC, then masking off the value of the control lines, so you're left with just the data.

You seem somewhat confused as to the direction in which the data goes (source -> destination) with the various instructions. Have you familiarised yourself with chapter 2 of 'the bible'?
http://www.semiconductors.philips.com/acrobat_d...UIDE_1.pdf

As for your question regarding the code byte: For every packet this is always the first byte received and should always be the first byte sent. So even if you're just sending one byte of data, the control byte must be included.

Hope this helps.

Regards,
Rob.