42
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 WAFER-C400EV.
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 ;Digital Output is 1001b
