Under Windows, it is possible (but not recommended) to open a serial port for output by using the following syntax:

open #1, "COM1\", output

Note that you must replace the normal colon in COM1: with a backslash. Otherwise, A-Shell thinks COM1: is an AMOS-type logical disk.

Unfortunately, it is not possible to open serial ports as input files.

The recommended approaches to serial I/O under Windows are either to use AutoLog (see above), or COMIO.SBR.