When trying to use WriteProperty to a 3rd Party BACnet device the following error is generated :
"An attempt has been made to write to a property defined as inaccessible through the BACnet protocol write service"
The error can be seen when using WriteProperty OR when attempting to Manually Command a BACnet Point on a 3rd Party Device
The property that is being commanded, is not available to be written to in the 3rd party device.
Many properties and object classes cannot be written to on BACnet devices, this is often defined in the devices PIC Statement which has hopefully also been BTL Tested.
Some BACnet devices allow writing to be enabled/ disabled, but this must be set up on the 3rd Party device itself depending on the site requirements. An example of this is in the Jace unit where there is a regular BACnet user that has no write capabilities from 3rd party devices, so to allow this requires a tick a box for "Super user".