Digi International Inc XBEEPRO2 Manual Do Utilizador

The XBee ZB 2x6x firmware includes an adaptive polling enhancement where, if an end device receives RF data 
from its parent, it sends another poll after a very short delay to check for more data. The end device continues 
to poll at a faster rate as long as it receives data from its parent. This feature greatly improves data throughput 
to end devices. When the end device no longer receives data from its parent, it resumes polling every 100ms.

End devices can be configured to send one or more IO samples when they wake from sleep. To enable IO 
sampling on an end device, the IR command must be set to a non-zero value, and at least one analog or digital 
IO pin must be enabled for sampling (D0 - D9, P0-P2 commands). If IO sampling is enabled, an end device 
sends an IO sample when it wakes and starts the ST timer. It will continue sampling at the IR rate until the 
sleep timer (ST) has expired. See chapter 8 for details.

If the commissioning pushbutton functionality is enabled (D0 command), a high-to-low transition on the AD0/
DIO0 pin (pin 20) will cause an end device to wake for 30 seconds. See the Commissioning Pushbutton section 
in chapter 7 for details.

Since an end device relies on its parent to maintain connectivity with other devices in the network, XBee end 
devices include provisions to verify its connection with its parent. End devices monitor their link with their 
parent when sending poll messages and after a power cycle or reset event as described below.

When an end device wakes from sleep, it sends a poll request to its parent. In cyclic sleep, if RF or serial data is 
not received and the sleep timer is not started, the end device polls one time and returns to sleep for another 
sleep period. Otherwise, the end device continues polling every 100ms. If the parent does not send an 
acknowledgment response to three consecutive poll request transmissions, the end device assumes the parent 
is out of range, and attempts to find a new parent.

After a power-up or reset event, the end device does an orphan scan to locate its parent. If the parent does not 
send a response to the orphan scan, the end device attempts to find a new parent.

Once all devices have joined a ZigBee network, the permit-joining attribute should be disabled such that new 
devices are no longer allowed to join the network. Permit-joining can be enabled later as needed for short 
times. This provides some protection in preventing other devices from joining a live network.

If an end device cannot communicate with its parent, the end device must be able to join a new parent to 
maintain network connectivity. However, if permit-joining is disabled in the network, the end device will not find 
a device that is allowing new joins.

To overcome this problem, ZigBee supports rejoining, where an end device can obtain a new parent in the same 
network even if joining is not enabled. When an end device joins using rejoining, it performs a PAN ID scan to 
discover nearby networks. If a network is discovered that has the same 64-bit PAN ID as the end device, it will 
join the network by sending a rejoin request to one of the discovered devices. The device that receives the 
rejoin request will send a rejoin response if it can allow the device to join the network (i.e. child table not full). 
The rejoin mechanism can be used to allow a device to join the same network even if permit-joining is disabled.

To enable rejoining, NJ should be set less than 0xFF on the device that will join. If NJ < 0xFF, the device 
assumes the network is not allowing joining and first tries to join a network using rejoining. If multiple rejoining 
attempts fail, or if NJ=0xFF, the device will attempt to join using association.

XBee routers and coordinators may require some configuration to ensure the following are set correctly:

•RF packet buffering timeout
•Child poll timeout
•Transmission timeout.