BACnet priority arrays and relinquish

A commandable BACnet object, such as a binary output that starts a fan or an analog value that holds a set point, does not simply take the last value written to it. A life-safety system, an operator and a schedule can all command one fan without fighting, because each writes to its own place in the object's priority array.

This guide is part of the BACnet and building protocols series.

How the array decides

Each of the 16 slots holds either a value or NULL (empty). The object's Present_Value comes from the first slot, counting from 1, that is not NULL. Priority 1 is the highest. When every slot is NULL, Present_Value takes the object's Relinquish_Default.

The standard names five priorities. The others are free for the site to assign:

A common site plan puts schedules at 16, energy management or demand response somewhere between 9 and 15, and operator overrides at 8. Write the plan down with the point list.

A worked example

A supply fan (binary-output 4) has a Relinquish_Default of inactive (off):

Two lessons follow. The DR system's command was accepted but had no effect while the operator held priority 8. And if the DR system had not released priority 12, the fan would have stayed off after the schedule came back on in the morning.

Relinquish, do not overwrite

To end a command, write NULL to the same priority. Writing the opposite value is not the same thing: the slot stays occupied, and it still blocks every lower priority. Forgotten commands at mid priorities are a common cause of plant that ignores its schedule weeks after a test.

Some clients cannot send NULL, or hide it behind another method. Beckhoff's documentation, for example, describes how its BACnet library writes NULL by sending a special value. Check how the client you use releases a command before you write anything.

Also check Minimum On/Off at priority 6. A binary output with minimum on and off times writes its own value at priority 6 for the minimum time after each change, so a lower-priority command may be delayed.

Test a write safely

1. Agree the object, the priority, the value, the duration and the release with the equipment owner.
2. Read the whole Priority_Array and Present_Value before the test, and record them.
3. Write the command. Read Priority_Array again: is your value in your slot?
4. Check Present_Value and the physical plant. If a higher slot holds a value, your command has no effect.
5. Release with NULL at the same priority, and read the array again.
6. Confirm that the plant has returned to its expected state.

A timeout on a write is an unknown result, not a failure. Read the array before you try again.

Priority and Modbus

Modbus has no priority array. A write changes the register, and the last writer wins. When a Modbus device is shared between a BMS and an energy system, agree which one may write, or use a controller that arbitrates. The BACnet vs Modbus guide compares the two.

Writes with Edge

Edge on the ZGW-20 Gateway can write to BACnet output and value objects that an administrator has provisioned for control. Each write goes to a chosen priority from 1 to 16, and priority 16 when none is set. Edge does not release a command with NULL, so agree how the command will be released, usually with the BMS tools, before a test. Treat every write as the approved control test above, and see the local control guide for the failure cases to plan.