How do the S4 Open Appliances support N2 Override and Release functions (KBA-01018-D2F4P8)?

Device Templates define point mappings in our S4 Open Appliances. For each defined point the Source String defines the point address on the N2 bus. Optional parameters .Value, .Override, and .Release force the N2 write transactions to be handled as a normal N2 Write, a N2 Override, or a N2 Release command. Read operations always return the N2 present value of a point.

Default actions if no parameter is specified are:
- N2 Override for physical I/O points.
- N2 Write for internal points.

Additional Comments
It is important to note that the address space created by our device type templates is independent from the configuration in the Metasys supervisory controller configuration database. Device Templates can be changed at will without impacting the operation of the Metasys head end.

The Metasys priority structure is implemented within the Metasys supervisory controller. Commands submitted to the N2 bus are executed in the sequence that they are submitted. i.e. on the N2 bus itself there is no priority structure.

However, there is a precedence between the four basic point operations.
- If a point is in Normal mode then it will accept and execute either a N2 Override or a N2 Write Command
- If a point is in Override mode then it will accept and execute another N2 Override Command. Important: It will also accept a N2 Write command but will not change the value of the point
- A N2 Release Command will release the override condition and put a point back into Normal operation
- A N2 Read Command will be processed at any time

The BACnet priority array mechanism is emulated in the N2 Router. If necessary, a N2 Release Command will automatically be generated when the last priority array entry is relinquished.

OPC does not have the equivalent of N2 Override or Release commands. You need to have separate points defined as aliases for the same N2 point address. One for each of the N2 Write, N2 Override, and N2 Release operations.

N2 Override and Release operations are not native to the System-91 devices. The S4 Open Appliances emulate this functionality for select points on the DX-9100 and other System-91 devices.

The final decision on what transactions a N2 device will process are determined by the programming in the device itself. Therefore, you will see some variation in operations depending on the type of device. As an example, we have seen some Vendor (VND) devices that will only accept N2 Override commands. You may also run into situations where interlocks or side loops are programmed into the N2 device logic that determine under what conditions a point can be modified.

Important Note
N2 devices do not care if a command that they are processing originated in a Metasys supervisory controller as a native N2 command or if it originated as an OPC or BACnet request and was translated to N2 by the S4 Open Appliance. One of the situations that our integration partners have experienced is that operators on a Metasys head end issue an override against a point which then causes normal N2 write operations initiated via OPC or BACnet to not affect the value of the target point. Careful coordination is required when operating with multiple head end systems in parallel.