High Quality Content by WIKIPEDIA articles! NMODEM is a file transfer protocol developed by L.B. Neal 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" family of computers. The block size was chosen to match the "natural" block size of a standard PC hard drive sector. This made buffering data for writing simpler.