Knowledge for Planning

Which Data Path Is Right?

The question isn’t which data path is the best, but which one is right for the system that receives the values. Often, the correct answer is: two of them in parallel.

Examining the gatewayExamine the signal path
JSONData Transfer Format
4Methods can be used in parallel
500Sensors per gateway
0 €License fee per sensor

A Comparison of the Four

Classification

All four protocols transmit measured values, but they do so based on different concepts of what a measured value is. If you overlook this, you’ll end up building devices that no one wants to maintain.

MQTT transports messages without concerning itself with their content. BACnet and KNX incorporate an object model, so they recognize the concept of a sensor value, including its unit and state. Modbus TCP recognizes only registers—that is, numbers at an address—with their meanings listed in a table alongside them.

PathModelThicknessLimit
MQTTMessages, Broker, JSONLean, supports many simultaneous recipients, suitable for analysis and dashboards.No object model. Meaning and units must be agreed upon.
BACnet/IPObjects according to ISO 16484-5The language of building automation. Values, units, and states are standardized.More complex to set up and assign addresses than MQTT.
KNXGroup Addresses, Data Point TypesWidely used in room automation; well-suited for lighting, shading, and individual room control.Designed more for actuators than for large amounts of data.
Modbus TCPRegisterSupported almost everywhere, easy to test, robust.No semantics. Every address mapping must be documented and maintained.

When It Makes Sense to Use Two in Parallel

The Typical Case

In many projects, the building automation system already uses one protocol, while the analytics system requires another. Control then runs via BACnet or Modbus into the existing control system, while the dashboard, analytics, and space analysis run via MQTT.

This isn’t a compromise, but a clear separation: Control requires a few values reliably and in real time, while analysis requires many historical values. Forcing both to use the same channel compromises the performance of one or the other.

The VL-700 BASE transmits the measurement values in JSON format to the MQTT broker of your choice. Other methods are also available. The key factors for the tender are who operates the broker and where it is located.

RegulationFew values, immediatelyThe Path to Building Automation. Reliability and response time matter—not the volume of data.
AnalysisMany data points, historicalThe path to the dashboard. Volume, time series, and analyzability matter—not milliseconds.
OperationWho operates the broker?This is the question that most often remains unanswered. It belongs in the request for proposal, not in the commissioning phase.

What's on the Topic

MQTT in Detail

The gateway publishes every received measurement value to the broker. The topic structure used is a system setting and is defined during setup—it cannot be changed during operation. There are three formats, and all three carry the same payload.

This means the question that arises before any proposal can be answered in advance: The integrator knows what structure to expect and can plan their part without having to wait for commissioning.

Can I retrieve the data via REST? Not yet: a REST interface is in the works. Currently, the data is sent out via MQTT, BACnet, KNX, or Modbus TCP—if your target system uses REST, please contact us for an update.

FormTopicWhen It Makes Sense
StaticA fixed topic for all devices on the gatewayThe standard case. One subscription is sufficient; differentiation occurs in the payload.
DynamicOne topic per device profile and device IDIf the target system wants to filter by topic instead of opening every message.
Set freelyexactly the agreed-upon string, without any appendixWhen an existing topic structure at the customer’s site must be maintained.
The payload contains: one JSON object per radio telegram, including the device profile, device ID, assigned name, timestamp, reception strength, and a list of measured values—multiple values per telegram if a device measures multiple parameters. The exact field description can be found in the VL-700 BASE manual.

What this means for your request for proposals

7 points
  • Specify the target system that will receive the values, including the manufacturer and version.
  • Specify the data path and, if two are to run in parallel, name both.
  • For MQTT, clarify: Who operates the broker, where is it located, and how is it secured?
  • For BACnet and Modbus, require the address and object lists as part of the scope of work.
  • Define the format and naming conventions for data points; do not leave this to chance.
  • Address historical data: Who stores it, for how long, and in what format.
  • Specify network segregation: Keep the field network and control network separate, with a firewall in between.
Order No. 11895VL-700 BASEEdge controller on a DIN rail, two separate networks, with a firewall in between.View →Signal path in six stepsThe SystemFrom measured value to building automation, with all the numbers.View →Software on the GatewayECSCommunicates with all four channels simultaneously and displays alarms and maintenance notifications in the browser.View →
Status and Sources

Information current as of September 16, 2026. Regulatory information has been compiled to the best of our knowledge and does not constitute legal advice. The text of the currently applicable version is always authoritative.

  • ISO 16484-5 (BACnet), data communication protocol for building automation.
  • ISO/IEC 14543-3 (KNX), part of the series of standards for home and building electronics.
  • MQTT and Modbus TCP: vendor-neutral industry standards.
  • Data transfer format and limitations as specified in the VL-700 BASE manual.

Would you like us to run the numbers for your building?

Send us the floor plan, dimensions, and target system. You’ll receive a recommendation for the number of units, a list of part numbers, and a quote.

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