Pc104 / ISA bus


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<p>I have a PC104 motion control board (homemade)which uses an analog device DAC8412 to output a +/- 10 volt control voltage. The chip has 4 DACs and a 12bit parallel data bus. Each dac is selected by addrsses lines A0 A1 00h= dac1 01h=dac2 10h=dac3 11h=dac4.

To write DATA to the DACs I use the following code:<pre>
mov dx,200h address dac1
mov ax,0800h mid count = zero volts
out dx,ax output data to dac
<p>Normally on an 8bit bus this instruction will output the low byte 00h to address 200h and the high byte 08h to address 201h (but 201h is DAC2).

<p>I understand that to direct the out instruction to put the 16bit word on the data bus the ISA control line IOCS16 should be held LOW during the write cycle. I have pulled this line down with a tri-state 74ls125 gate but I still get a