source code for updating firmware remotely

[Mark]

Bootloaders work by going into a "special mode" . They are usually confined to a block of memory that never gets updated. It doesn't really matter how much RAM your program uses because all of the RAM is available to the bootloader since the normal program is not running. Also goes the same for your RS485 routines. The bootloader will have its own receive and transmit routines, usually not interrupt driven. Blocks of data are sent to the PIC, the PIC then writes these values. The PIC could then send back an ACK to signal the host that the writing is complete. Take a look at Microchip's App note http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en012031 it also gives you the ability to read the data back to verify what is there.