Can I use an analog input for a discrete signal field device on the A-B Micro820?
To elaborate on @AltAir's comment, the Micro820 controller you have has 12 inputs, 4 of which (I-00 - I-03) can be used as either 0-10v analog inputs or 24VDC digital inputs.

There are a few considerations:


Unlike the other digital inputs (I-04 - I-11), the shared digital/analog inputs are not isolated from the controller's power supply.  The common for these inputs is tied to the power supply common.
These shared inputs, when used as digital inputs, can only be used in a sinking wiring configuration.  (The other digital inputs can be wired either as sourcing or sinking.)


For wiring diagrams and technical specifications, see the manual.

Alternatively, your controller has 2 expansion slots for plug-in modules, so you could add more discrete inputs by adding an IQ4, for example.