Serial Communications - Tandy 1000 SL Technical Reference Manual

Software
Serial Communications
These routines provide asynchronous byte stream I/O from and to the
RS-232C serial communications port. This device is labeled the
auxiliary (AUX) I/O device in the device list maintained by MS-DOS.
Software Interrupts
14 hex (20 dec)
Function Summary
AH
=
0: Reset Commport
AH
=
1: Transmit character
AH
=
2: Receive character
AH
=
3: Get current Comm status
DX
=
communication port number (0 or 1)
20.