The Data Expiration configuration on the Ethernet Gateways determines how long sensor readings will be present in the data registers before the data is cleared out. This applies to the MODBUS TCP and SNMP Interfaces of the Ethernet Gateway. Readings from each individual sensor will be stored in the data registers until one of the following events occurs:
- Sensor reports new data overwriting the previous data.
- The gateway reboots (which will clear out data registers for all sensors on the gateawy’s Sensor Network).
- Data Expiration timer expires.
Note: If the data register is cleared out, it can be easy to mistake this for a “0” reading. For example in the case of a temperature sensor, you might poll the data registers and mistakenly interpret the empty register as “0” or “0°C/32°F”.
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The most common way in which this feature is used is to demonstrate inactivity of a sensor (the Data Expiration timeout is configured in the scale of hours). For example, if you have configured your sensor to have a 60 minute Heartbeat, you might enter the Data Expiration as 3 hours (3 X Sensor Heartbeat). Therefore if the sensor loses connection for 3 Heartbeats (3 hours), the registers will be cleared, at which point you would investigate the sensor to determine what caused the inactivity. Using this in conjunction with Data Age in the Sensor Registers can give you a clear picture of inactivity issues.