Groups and Tags

A tag is one address inside a device, with read/write attributes, a data type, and a precision. A group is a set of tags with its own polling interval.

The group is the unit of collection, reporting, and subscription: every tag in a group is polled at the same interval, packed into a single message, and published together; northbound applications also subscribe per group. How you divide tags into groups therefore determines the shape of the published payload, its size, and the bandwidth it consumes — decide on grouping before configuring tags.

Grouping strategy

Group by polling interval. This matters most. Temperature and level readings change slowly and a five-second interval is plenty, while vibration or current may need 100 ms. Putting them in one group polls the slow values at the fast rate — loading the device unnecessarily and filling every payload with values that have not changed.

Group by destination. Northbound applications subscribe per group. If some tags go to the cloud while others are only read by on-site SCADA, put them in separate groups so each can be subscribed independently.

Group by device or process section. Runtime statistics (group_last_timer_ms, group_tags_total) are reported per group, so grouping that mirrors the plant structure makes problems easier to locate.

Keep groups a reasonable size. All tags in a group are published in one message, so a larger group means a larger payload. After the driver runs, check group_last_timer_ms: if it approaches or exceeds the group's interval, the poll cannot finish within one cycle and the group should be split or the interval widened.

Limits: at most 512 groups per southbound driver, and a minimum polling interval of 100 ms. For the full list, see Configuration specification.

Create a group

Click the driver node to open its group list, then click Create Group:

Field	Description
Name	Group name, up to 128 characters. It appears in the default upload topic and in OPC UA NodeIds
Interval	Polling and reporting interval for the group, in milliseconds, minimum 100

Add tags

Open Tag List on the group, then click Add Tag:

Field	Description
Name	Tag name, up to 128 characters
Attribute	read, write, subscribe; multiple may be selected — see Tag attributes
Type	Data type. Supported types differ per driver; see the relevant driver page
Address	The format differs per driver. In Modbus, 1!40001 means slave 1, holding register 40001
Decimal	Optional — see Shaping the data
Bias	Optional — see Shaping the data
Precision	Configurable for float and double, range 0–17
Unit	Optional engineering unit for the tag, such as ℃ or kPa. A label only — it takes no part in the conversion
Description	Optional, up to 256 characters

Tag attributes

Attribute	Behavior
read	Polled at the group's interval
write	Allows writes from a northbound application, the data monitoring page, or the HTTP API. Without it, a write returns an error
subscribe	Sends a message to northbound applications only when the value changes

Attributes can be combined. A tag that is both collected and controlled needs read and write together.

subscribe suits discrete and status tags that rarely change: the device is still polled at the group's interval, but a message is produced only when the value moves, which cuts uplink traffic significantly. For example, a tag that defaults to 0 publishes only when it changes to 2:

Shaping the data

The raw value in a device is often not the value the business needs — a register may hold an integer scaled by ten, or a baseline may have to be subtracted. This conversion can happen at collection time so downstream systems receive usable values directly.

Parameter	Formula	Constraints
Decimal	device value × decimal = reported value	Applies only when the tag attribute is read
Bias	device value + bias = reported value	Applies only when the tag attribute is read; unavailable when the tag also has write; numeric and float types only

Precision

Precision is configurable for float and double types, range 0–17:

• With no precision set, five decimal places are kept by default.
• From the second decimal place on, two consecutive 00 or 99 digits trigger rounding — 1.02990 displays as 1.03, and 1.80012 as 1.8.
• Once a precision is set, EMQX Neuron no longer applies that rounding.

TIP

For more involved conversion, filtering, or aggregation — a formula across several tags, or an average over a time window — use SQL in Data Processing.

Verify

Once tags exist, the driver node should move to Running and Connected. If it stays Disconnected, see Diagnosing a connection.

The Add Tag page can read a tag directly to confirm the address is correct (Modbus TCP driver only):

With the configuration in place, open Data Monitoring and Device Control to see live values. For large tag counts, use Excel import instead of entering them one by one.