Tandy 1000 HX Technical Reference Manual page 406

Line Printer
These routines provide an interface to the parallel line printer.
This device is labeled "PRN" in the device list maintained by the
operating system.
Software Interrupts:
17 hex (23 dec)
Function Summary:
AH = 0: Print Character
AH = 1: Reset Printer Port
AH = 2: Get Current Printer Status
Function Descriptions:
Print Character
Print a character.
Entry Conditions:
AH = 0
AL = character to print
DX = printer to be used (0-2)
Exit Conditions:
0AH = printer status; see Get Current Printer Status (AH = 2)
below
(If bit 0 is set, the character could not be printed
because of a timeout error.)
Reset Printer Port
Reset (or initialize) the printer port.
Entry Conditions:
AH = 1
DX = printer to be used (0-2)
Exit Conditions:
AH = printer status; see Get Current Printer Status (AH = 2)
below
16