**Mandatory.** Must be implemented by all level I and Level II devices.

This is intended for nodes that have been initiated, is part of the segment and is powered up. All nodes that have a nickname-ID that is not set to 0xFF should send this event before they go on line to do their “day to day” work.

Normally all nodes should save their assigned nickname-ID in non-volatile memory and use this assigned ID when powered up. A segment controller can however keep track of nodes that it controls and reassign the ID to a node that it did not get a new node on-line event from. This is the method a segment controller uses to detect nodes that have been removed from the segment.

For the nickname discovery procedure this event is used as the probe. The difference between a probe and a new node on line is that the later has the same originating nickname as value in byte 0.

If a node send this event with the unassigned ID 0xFF and byte 0 set to 0xFF it has given up the search for a free ID.

It is recommended that also level II nodes send this event when they come alive. In this case the target address is the 16-byte data GUID of the node with MSB in the first byte. 

Standard form (Mandatory)

 | Data | Description | 
 | :----: | ----------- | 
 | 0    | **Target address**. This is the probe nickname that the new node is using to test if this is a valid target node. If there is a node with this nickname address it should answer with probe ACK. A probe always has 0xff as it's own temporary nickname while a new node on line use a non 0xff nickname. | 

 Extended Form for node with 16-bit nickname. (Mandatory for nodes with 16-bit nickname)

 | Data | Description | 
 | :----: | ----------- | 
 | 0    | **Target address**. This is the MSB of the probe nickname that the new node is using to test if this is a valid target node. If there is a node with this nickname address it should answer with probe ACK. A probe always has 0xff as it's own temporary nickname while a new node on line use a non 0xff nickname. | 
 | 1 | LSB of probe nickname. |

On a Level II system.

 | Data | Description | 
 | :----: | ----------- | 
 | 0-15 | **GUID**. This is the GUID of the node. MSB in byte 0. | 

