Ampro Corporation P5X manual Using the Control Lines for Additional I/O

;----------------------------------------------------------

; Code to set the parallel port mode to bi-directional

;----------------------------------------------------------
MOV	AH,0CDh	; AMPRO		command
MOV	AL,0Ch	;	AMPRO	function
MOV	BX,01h	;	Extended mode		(use 00 to set output-only mode)
INT	13h

Within bi-directional mode, the port can be in its input state or output state. The code shown above leaves the port in its input state. An IN instruction of I/O address 378h reads the current state of the data lines.

To change the port between input and output states, write a 1 to bit five of the control register to set the port to its input state; or a 0 to set it to its output state. Here is a code sample for dynamically changing the port direction (after it is in Extended Mode).

;----------------------------------------------------------

; Code to change the parallel port direction to input

;----------------------------------------------------------
MOV	DX,37A
IN AL,DX
OR AL,20h		;set bit 5 (input)
OUT	DX,AL
;

;----------------------------------------------------------

; Code to change the parallel port direction to output

;----------------------------------------------------------
MOV	DX,37Ah
IN AL,DX
AND	AL,0DFh	;clear bit 5
OUT	DX,AL

Using the Control Lines for Additional I/O

Besides the eight data lines, you can use the four control lines (STB*, AUTOFD*, INIT*, and SELIN*) as general purpose output lines. Similarly, you can use the five status lines (ERROR*, SLCT, PE, ACK*, and BUSY) as general purpose input lines.

You can read the four control lines and use them as input lines. These lines have open collector drivers with 4.7K ohm pull-ups. To use a control line as an input line, you must first write to its corresponding bit in the control register. If the line is inverting (*), write a 0, otherwise write a 1. This will cause the line to float (pulled up by the 4.7K ohm resistors). When a line floats, you can use it as an input.

Enabling the Parallel Port Interrupt

Bit 4 in the Control Register enables the parallel port interrupt. If this bit is high 1, then a rising edge on the ACK* (IRQ) line will produce an interrupt on the parallel port interrupt, IRQ7.

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