The BACnet object goes into fault when writing an NaN to an MNB controller.
TAC I/A Series
MNB-V1, MNB-V2, MNB-70 AND MNB-300 BACnet controllers
Firmware 1.424 and higher responds to receiving an NaN with an Error: "value out of range". In firmware 1.420 it would accept the NaN and respond with a normal acknowledge message. This is a result of a change required for BTL compliance.
A slight set up change can accomplish the same result without causing the object to go into fault. In the ENC edit the properties of the AVSPP object and set the fallback to "null". Go to the slot sheet of the object being overridden to hide the "Set" option. This will prevent the operator from attempting to send an NaN that causes the fault. When a right click override is performed only the emergency (priority 1) and Override (priority 8) choices are displayed. When the override expires or the object is manually put back into “auto” the “fallback” value is output which is “null” and that is accepted at the controller and serves the same function as sending an NaN.