Tandy 1000 HX Technical Reference Manual page 404

Transmit Character
Transmit (output) the character in AL (which is preserved).
Entry Conditions:
AH = 1
AL = character to transmit
DX = port number (0 or 1)
Exit Conditions:
AH = RS-232 status; see Get Current Comm Status (AH = 3)
below (If bit 7 is set, the routine was unable to transmit
the character because of a timeout error.)
AL is preserved.
Receive Character
Receive (input) a character in AL (wait for a character, if neces-
sary). On exit, AH will contain the RS-232 status, except that
only the error bits (1,2,3,4,7) may be set; the timeout bit (7), if
set, indicates that data set ready was not received and the bits
in AH are not meaningful. Thus, AH is non-zero only when an
error occurred.
Entry Conditions:
AH = 2
DX = port number (0 or 1)
Exit Conditions:
AL = character received
AH = RS-232 status; see Get Current Comm Status (AH = 3)
below
Get Current Comm Status
Read the communication status into AX.
Entry Conditions:
AH = 3
DX = port number (0 or 1)
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