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Appendix D: Digital I/O
One of the characteristics of digital circuit is its fast response to high or low signal.
This kind of response is highly needed for harsh and critical industrial operating
environment. SAGP-648E is designed with 4-bit digital inputs and 4-bit digital
outputs.
There are two kinds of signals (Input and Output) used by the Digital I/O function.
These signals are used to control external devices that need On/Off circuit or TTL
devices. When one of the signals has been selected, the user can read or write
data to the system through the Digital I/O function.
A BIOS function call (INT 15H) is used to control the Digital I/O Function:
INT 15H:
AH – 6FH
Sub-function:
AL – 8: Set the Digital port as INPUT
AL: Digital I/O input value
Example program:
MOV AX, 6F08H ;setting the Digital port is input
INT 15H ;
AL low byte = value
AH – 6FH
Sub-function:
AL – 9: Set the Digital port as OUTPUT
BL: Digital I/O output value
Example program:
MOV AX, 6F09H ;setting the Digital port is output
MOV BL, 09H ;Digital value is 09H
INT 15H ;
Digital Output is 1001b
