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Understanding Wireless Sensor Link Mode

Keywords: Link Mode, Connection, Start Up


Information on how a sensor connects to the network.

When the batteries are inserted into a sensor, it tries all of the available channels looking for a gateway. This is called “Link Mode”. The sensor starts on the first channel and sends a message “I’m number 12345, can I talk to you?”. If it doesn’t hear anything back it assumes there is not a gateway on that channel and tries the second channel. After it has tried all of the channels and determines that there is no available gateway, it stops sending messages for two hours. Each scan cycle takes about 30 seconds but uses over a day’s worth of battery, so if it just continued to scan continually when the gateway wasn’t powered on it would burn out its battery very quickly. After two hours it starts the process over again starting at the first channel. Similarly, if for instance the sensor was talking to the gateway on channel 15, but has multiple failed transmissions, (2 heartbeats that each have a report plus two retries, 6 missed communications in a row) it assumes the gateway it was communicating with is no longer available and enters the same “Link Mode”.

The reason this is important is because if the battery is put in the sensor before the gateway is active and ready to listen for the sensor, then the sensor will miss the gateway and not scan again for it for two hours. In order to get the sensor to scan again more quickly you must remove the batteries for at least 60 seconds allowing the processor to completely un-power, then put the batteries back in allowing it to start scanning right away. If the batteries are only out for a few seconds the processor is still running off of capacitance on the board and doesn’t “re-boot” when the batteries are put back in.

In order to have the best start up experience make sure the gateway is powered on and active, before you put the batteries in your sensors so they can boot up and find the gateway on the first scan. If the batteries are in the sensors before the gateway has gone active then it will take two hours for them to join the network and start reporting their readings.

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