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Appendix D. 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-E668EV.
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 Watchdog
Timer:
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
