Tandy 1000 HX Technical Reference Manual page 403

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 main-
tained by MS-DOS.
Software Interrupts:
14 hex (20 dec)
Function Summary:
AH = 0: Reset Comm Port
AH = 1: Transmit Character
AH = 2: Receive Character
AH = 3: Get Current Comm Status
DX = communication port number (0 or 1).
Function Descriptions:
Reset Comm Port
Reset (or initialize) the communication port according to the
parameters in AL, DL, and DH.
Entry Conditions:
AH = 0
AL = RS-232C parameters, as follows:
DX = port number (0 or 1)
7 6 5
Baud Rate
4 3
Parity
2
Stop Bits
1 0
Word Length
13
000 = 110 baud x0 = none 0 = 1 bit 10 = 7 bits
001 = 150 baud 01 = odd 1 = 2 bits 11 = 8 bits
010 = 300 baud 11 = even
011 = 600 baud
100 = 1200 baud
101 = 2400 baud
110 = 4800 baud
111 = 9600 baud
Exit Conditions:
AX = RS-232 status; see Get Current Comm Status (AH = 3)
following