Is this a serial RF interface? If it is low speed, then I would use a Holtek encoder (www.holtek.com and find the right IC, then go to www.findchips.com and see if it is available in distribution)
You can do it the hard way and use a microcontroller's UART to do this. You need to be able to generate start and stop bits, and a bit or error detection (hamming code, crc) wouldn't hurt.