Serial line · electricity

Electricity meter reading over RS-485 and Modbus

Electricity meters speak a different language from heat and water meters. An RS-485 serial line with the Modbus protocol is the standard way to bring unit and common-area electricity meters into the same panel.

At a glance

Physical interface: RS-485 — two-wire differential serial line
Protocol: Modbus RTU (address + function code + registers)
Topology: Daisy chain with termination at the ends
Addressing: Unique Modbus address per device on the line

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What are RS-485 and Modbus?

RS-485 is a serial communication interface widely used in industry. Its two-wire differential design works reliably over long distances and in electrically noisy environments — which makes it a natural fit for switchboards and building installations.

Modbus is the language spoken over that line. In Modbus RTU the master asks a device by address and function code, and the device answers with register values. It is simple, well documented and supported almost everywhere.

Most electricity meters expose their measurements as Modbus registers. Which register holds which quantity is defined in the device's register map; once that map is known to the system, the meter becomes readable.

How it works

Reading with RS-485 / Modbus, step by step

The path from installation to an index landing in the panel.

The line is installed
A two-wire RS-485 line runs from the electricity meters to the switchboard. Devices are connected in a daisy chain, with termination resistors at the ends where required.
Addresses are assigned
Each meter gets a unique Modbus address on the line. Two devices sharing an address cause collisions, so this is verified during installation.
The register map is defined
The system is told which register holds which quantity: active energy, reactive energy, voltage, current, power and power factor.
The reading round runs
The master polls meters on the line in turn; returned register values are converted into engineering units and split into index and instantaneous values.
Data lands in the archive
Electricity indexes are archived alongside heat and water meters and used together in cost allocation.

Technical summary

Physical interface: RS-485 — two-wire differential serial line
Protocol: Modbus RTU (address + function code + registers)
Topology: Daisy chain with termination at the ends
Addressing: Unique Modbus address per device on the line
Quantities read: Active/reactive energy, voltage, current, power, power factor
Use: Unit and common-area electricity meters

Choosing

When is RS-485 / Modbus the right choice?

There is no single right path for every building. These two lists are the criteria we apply during a site survey.

Good fit

Projects where unit electricity is metered separately
Sites tracking common-area consumption
Buildings where switchboard installation is practical
Projects that want heat, water and electricity on one statement

Poor fit

Older electricity meters with no Modbus output
Scattered measurement points where a line cannot be run

In NovuSoft

How RS-485 / Modbus reading is managed in the panel

Supporting a protocol is not enough; what the data means and what happens when a meter cannot be read must be defined too.

Four meter types in one panel

Electricity meters are managed in the same project and the same list as heat, hot water and cold water meters, and appear as a separate line on the cost allocation statement.

More than the index

Beyond active and reactive energy indexes, voltage, current, power and power factor can be read and stored.

Common-area tracking

Common-area meters are defined separately so shared consumption never blends into unit consumption.

Telling line faults apart

Every device on a line going silent is a different pattern from one device going quiet — the panel makes that distinction visible.

Frequently asked questions

Questions about RS-485 / Modbus

Can any electricity meter be read over RS-485?

No. The meter needs a communication port that supports Modbus. Devices without one require either replacement or a suitable interface unit; the site survey establishes which.

How many meters can share one line?

It depends on device load, cable quality and line length. Where capacity is exceeded the line is split or a repeater is added. The installation plan comes out of the survey.

How does electricity enter the heat cost statement?

Electricity appears as its own line item on the allocation statement. Heating and domestic hot water are allocated by regulation rules, while electricity is distributed by measured consumption.