the specs for x,y,zmodem can be found here:

http://www.techfest.com/hardware/modem/xymodem.htm

I think xmodem is the easier one to implement.

an assembler source (for an 8080 CP/M system if I remember correctly) can be found here:

as it is 8080 based, it should be fairly readable to an 8051 knowledgeable person.

http://www.retroarchive.org/cpm/cdrom...ODEM32.ASM


good luck
Patrick