If the 9403 is already there and you have a spare DI and a spare DO , I would use an optocoupler (CNYxx) and two resistors for monitoring. The DO serves as a ~5V 2mA powersupply and together with a 3.3k resistor and the coupler output form the DI input.
The 12V signal passes a 5.6k (or 10k) resistor as a current limiter to the input (LED) of the coupler.
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'
