BACnet and Modbus are supported. Why can the equipment still not be integrated?

The unit supports BACnet, the heat pump supports Modbus, and the supervisory system can work with both. It seems all that remains is to connect the equipment.

But during commissioning, it turns out that the temperature can be read, the setpoint cannot be written, detailed alarms are unavailable and some values have unclear scaling. A shared communication language does not mean a device provides all the information needed or executes every required command.

What does the protocol guarantee?

BACnet defines the exchange of automation objects, properties and services. Devices may implement different parts of the standard depending on their purpose. Two BACnet devices therefore do not have to offer the same functionality. ¹

Modbus provides a way to exchange discrete signals and register data. But the meaning of a particular register must be checked in the equipment documentation. The Modbus label alone does not explain which parameter a received number represents. ²

Before selecting a gateway, we therefore need more than the protocol name: we need a list of the information and actions required.

BACnet: checking the implemented capabilities

For BACnet, it is worth requesting the PICS — the manufacturer’s statement of implemented capabilities. It describes supported services, objects and, in particular, writable properties. ³

BTL certification is useful for checking protocol interoperability. But it does not replace the project’s functional specification: whether the exact setpoints, alarms and status information required are available. ⁴

For example, one application may need only a start command and a common fault alarm. Another may require the actual operating mode, temperature limits, communication status and command confirmation. These are different integration scopes, even if both quotations say ‘BACnet connection’.

Modbus: a number without a description proves nothing

The register map should be checked for addressing, data type, units, scaling and read/write permissions. Schneider Electric, for example, specifically explains the difference between a register address and a register number, and the use of a scaling factor. ⁵

A hypothetical temperature register returns 235. If the documentation specifies a scale of 0,1 °C per unit, this means +23,5 °C. Without that description, it cannot be assumed that the number should be divided by ten.

For multi-register values, the word order must also be checked against the documentation. Even a plausible number can be decoded incorrectly: an error does not always appear as an obvious reading of thousands of degrees.

A gateway cannot create a missing function

If the equipment does not transmit measured electrical power, converting Modbus to BACnet will not create it. Separate metering can be connected, or a calculated estimate developed, but these must be labelled differently.

Likewise, an accepted write does not prove that a command has been executed. In BACnet, the action may be determined by priorities, local control and device limitations. The specific behaviour is described in the controller documentation. ⁶

During a test connection, we would check not only reading, but also an authorised change, its confirmation and a return to normal control logic — without dangerous experiments with protective functions on an operational site.

What document does the client need?

An agreed points schedule is needed: parameter name, equipment, address or object, unit, access permissions, update frequency and response to loss of communication. Licences and integration limitations should be listed separately.

This allows proposals to be compared and makes it clear in advance exactly what will remain unavailable.

An open protocol is a good foundation. Integration is complete when every required parameter is read correctly and every permitted command has been tested on the actual equipment.