> If you have a file containing the NULL, it can be copied from the keyboard
> via the DOS command  COPY TEST.FIL /b > COM1  The /b invokes a binary copy
> mode thereby allowing the NULL to be sent/copied where ever. The MODE
> command will set up the com port for bits, baud etc.

On both the PC BIOS and DOS, a null character code is used to flag special
keys.  In the case of the BIOS, a key such as an arrow will return a char-
acter code of 0 and a scan code indicating what key was pressed.  DOS will
normally ignore scan codes, but if a key's character code is zero, the key
will be registered as a zero byte followed by the scancode byte.

The "null" key (ctrl-2) has a character code of 0 and a scan code of 3.  A
terminal program which is specifically set to look for this key will be
able to process it, but without special coding the key will not work.  DOS
copy CON: won't work with ^@ (it will be placed in the file as ^@^C).