NMODEM  NMODEM is a file transfer protocol for bulletin board systems (BBSs) developed by L. B. Neal, who released it in 1990. NMODEM is essentially a version of XMODEM-CRC using larger 2048 byte blocks, as opposed to XMODEM's 128 byte blocks. NMODEM was implemented as a separate program, written in Turbo Pascal 5.0 for the IBM PC compatible family of computers. The block size was chosen to match the common cluster size of the MS-DOS FAT file system on contemporary hard drives, making buffering data for writing simpler.