Meter count alone is not line capacity
Wired M-Bus connects meters through a two-wire bus to a master or converter. A nominal “64-meter converter” figure is only a starting point. Actual segment capacity depends on the master’s current budget, each meter’s unit-load rating, loop resistance, total cable capacitance, communications speed and address management. Wired M-Bus and RS485 Modbus are distinct interfaces and must not be connected directly as if they were interchangeable.
Understanding unit loads (UL)
In common M-Bus load calculations, 1 UL corresponds to 1.5 mA; some endpoints consume more than one unit load. The basic planning calculation is I_budget = Σ(UL_i × 1.5 mA). Check the exact converter’s output rating, required voltage margin, installation conditions and expansion reserve instead of assigning every device a default 1 UL.
Example: sixty meters in a building
If all 60 meters draw 1 UL, the aggregate nominal load is 60 × 1.5 = 90 mA. If each draws 2 UL, that becomes 180 mA. A notional 20% expansion reserve brings the planning budgets to at least 108 mA and 216 mA, respectively. These values demonstrate the method; they are not stated capabilities of an ATLAS converter. Verify the selected product’s published master ratings and permitted segment arrangement.
Cable resistance and capacitance are separate constraints
The round-trip resistance of the route to the farthest meter causes a voltage drop. The combined capacitance of the trunk, branches and connected devices limits communication margin, especially at higher speeds. For instance, 0.8 km of cable rated at 100 nF/km contributes about 80 nF before device and accessory capacitance is added. This is only an estimate; field measurements and the manufacturer’s limits govern final acceptance.
Baud rate and address management
EN 13757-2 establishes 300-baud support for wired M-Bus, while 2400 baud and other rates may be supported by individual products. Faster communication may be less robust on long, highly capacitive segments. Assign unique primary addresses and verify unique secondary identification data. Address collisions can produce intermittent or misleading readout faults.
Six commissioning checks
Record every meter’s UL requirement and the actual master’s UL/mA capacity.
Calculate conductor size, farthest loop resistance and the length and capacitance of all branches.
Compare bus voltage at the master and farthest meter with the product-specific limits.
Scan for primary/secondary address conflicts and retain the serial-to-address map.
Log response rates and timeouts at 300/2400 baud where supported.
Test disconnection/reconnection, power interruption and recovery of readout.
Matching converter to the application
ATLAS M-Bus converters and data collectors can be reviewed for different building architectures. The final design should rely on the selected model’s documented output capacity, meter loads, isolation and upstream interface. Do not treat a nominal device count as a guarantee for an unmeasured cable layout.
Project support: Provide meter counts, a UL list and a cable drawing to request a documented ATLAS M-Bus equipment selection.
Kaynak başlıkları / References: EN 13757-2:2018+A1:2023, Siemens M-Bus technical manual (2025)