The Print Screen service assumes that the serial port has already been programmed with the appropriate parameters (Serial Power On, Wired mode, BAUD Rate, parity, etc.) to communicate with a serial printer. If this hasn't been done, use the DOS "Mode" command to establish the proper settings. See also Int 14h, Fcn 00h (page 3-91), Int 15h, Fcn 49h and Int 15h, Fcn 4Ah for more detail.