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Appendix B. Digital I/O
One characteristic 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. That’s why we design
4-bit digital inputs and 4-bit digital outputs on the NOVA-7800.
Digital Input and Output, generally, are control signals. You can
use these signals to control external devices that needs On/Off
circuit or TTL devices. You can read or write data to the
selected address to enable the function of digital IO.
A BIOS function call (INT 15H) is used to control the Watchdog
Timer:
INT 15H :
AH – 6FH
Sub-function:
AL – 8
: Set the Digital port is 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 is 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 ;
