Understanding the Gateway Sensor List

Sensor Lists are the manner by which sensors receive authorization to send data to gateways. Sensor Lists are the list of sensor ID’s stored on a gateway that authorize the gateway to acknowledge a Join request from a sensor, allowing that sensor to link to the gateway. Sensor Lists are constructed in different ways. This article will provide details on how the Sensor List of gateways are assembled.

Sensor Lists from Monnit software

The most common manner by which Sensor Lists are constructed is through networks in Monnit software. When a gateway and a sensor are added to the same Sensor Network in Monnit software, the gateway downloads all of the sensor ID’s added to that Sensor Network to the gateway’s Sensor List. If a sensor ID is on a gateway’s Sensor List, it is authorized to receive transmission from that sensor. Therefore if a sensor is powered on within range of a gateway which has received the sensor’s ID onto its Sensor List, the sensor will successfully hand shake with the gateway and begin to deliver data to the gateway and subsequently to the iMonnit Online Portal. For information regarding how sensors link to gateways, see the following article. Link Mode Explained

When gateways download sensor ID’s to their Sensor Lists

Note: Sensors lists are sent from Monnit software to the gateway, but they are not sent from gateways to Monnit software. Therefore if you add sensors to a gateway’s local interface, the sensors ID’s are not synchronized to the Monnit software.

An important detail to this operation is that gateways do not download sensor ID’s to their Sensor Lists immediately after they are added. There are certain scenarios which trigger a gateway to download sensor ID’s; these scenarios are listed below.

  • When a gateway is first powered up
  • When the button on the gateway is pressed and immediately released while the gateway is already communicating with the Monnit software (Run Mode)
  • When a Reform command is performed on the gateway (see the following article regarding Reforming gateways: Reform Gateway Network )

    A Reform of a gateway’s Sensor List occurs in three scenarios: manually sending the Reform command from the software, resetting the gateway to default settings, or when the gateway’s firmware is upgraded.

  • Every 12 hours by default

Sensor ID’s remain on the gateway’s sensor list until a Reform

It is important to know that gateways maintain sensors on their sensor lists indefinitely unless the gateway’s network is Reformed. Therefore if a sensor was added to the same Sensor Network as a gateway, the gateway will maintain that sensor ID on its list even if the gateway is moved to a different Sensor Network. Sensor sensor ID’s are only removed if the gateway’s network is Reformed, if you move devices between networks, it is important to Reform the networks of any gateways that are moved to different Sensor Networks. Otherwise, you may find a sensor communicating with a gateway not on its same Sensor Network if it had previously been on the same Sensor Network and the gateway has not received a Reform command.

The Sensor List shown in the iMonnit Online portal

When viewing a gateway through iMonnit Online, there is a tab called “Sensors”. While it is common to assume this list contains all the ID’s of sensors that are authorized to communicate with the gateway, it is important to know this is not the case. This list contains the ID’s of sensors that have last successfully communicated with the gateway. Therefore this is not the feature by which you can determine which sensor ID’s are authorized to communicate with the gateway. In order to see the sensors that are actually on the gateway’s Sensor List, you would need to access the gatway’s Wireless Sensor Network through the gateway’s HTTP Interface (Ethernet Gateway only).

Sensors tab in iMonnit Online
Sensors tab in iMonnit Online

Adding sensor ID’s to a gateway’s Sensor List using the HTTP Interface (local configuration page)

The Ethernet Gateway offers a HTTP Interface which you can access through a browser to configure the gateway. This interface allows a user to view the gateways Sensor List (Wireless Sensor Network), add sensors, or Reform the network (erase the Sensor List). For information accessing the HTTP Interface of an Ethernet Gateway, see the following articles.

Ethernet Gateway 4 - Accessing local configuration page (HTTP Interface) Locally

Enabling Local Configuration Page (HTTP Interface) of the Ethernet Gateway 4 through iMonnit Online

The HTTP Interface of an Ethernet Gateway will list which sensors are on the gateway’s Sensor List and are authorized to communicate with the gateway. There is also a slot associated with each sensor ID which can be useful when implementing the Ethernet Gateway’s MODBUS TCP Interface.

HTTP Interface - Wireless Sensor Network
HTTP Interface - Wireless Sensor Network

Important behaviors of how the gateway’s Sensor List forms on the Ethernet Gateway

For customers using the MODBUS TCP or SNMP Interfaces on the Ethernet Gateway, there may be some important behaviors to understand when sensor ID’s are assigned to the gateway’s Sensor List.

  1. When sensor ID’s are downloaded to the gateway’s Sensor List for the first time from Monnit software (as opposed to adding them manually through the HTTP Interface), they are arranged into the slots by numeric sequential order. In other words, a sensor ID “116541” would be arranged in the slot before sensor ID “116542”. Therefore if your sensors are arranged in a specific slot arrangement, this arrangement can change if your sensors are not in sequential order already and you Reform your network through the software, they may be arranged in a different slot arrangement.
  2. There is an option to choose which slot you assign a sensor to on the gateway’s Wireless Sensor Network list. If you insert a sensor to a specific slot, any sensors after that slot are moved up a slot.
  3. There is no manner by which to remover a single sensor from the gateway’s Sensor List. Therefore if you need to assign a specific sensor to a specific slot, you will either need to use the gateway’s HTTP Interface to assign that sensor to a specific slot, or Reform the network completely and assign each sensor manually to the desired slot.


There are specific behaviors of the gateway’s Sensor List that might determine how you operate, especially when using the gateway’s HTTP Interface to manually assign sensors to the gateway’s Sensor List. If you need additional guidance, please contact support@monnit.com.

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