INT 14 - Digiboard - CCB COMMAND - SET TRANSMIT MID-WATER MARK                  
	AX = 164Dh
	BX = new mid-water mark
	CL = byte 1
	DX = port number (00h-03h) (04h-43h for XAPCM232.SYS)
Return: AH = status
	    00h successful
	    80h error
	    FFh error
SeeAlso: AH=16h"Digiboard",AX=164Ch,AX=164Eh,AX=164Fh