A wired M-Bus master can read many meters over one pair, but address capacity is only one boundary. Commissioning also has to prove unit load, remote voltage, cable capacitance and topology, meter identity, exact telegram records, quality and reconciliation with the local display.
Inventory the segment
Record every meter's manufacturer, exact model, firmware where exposed, M-Bus unit-load requirement, primary address, secondary identity, supported baud rate and required records. Mark planned spare capacity separately from installed devices.
Primary addresses 1 through 250 are ordinary assignments. Address 0 is used for an unconfigured slave; values 251 through 255 have special, selection or broadcast roles rather than serving as ordinary unique addresses. Use the primary-address schedule checker to expose duplicates, count mismatches and special values.
Calculate electrical margins
Add the unit loads of every slave and compare the total with the master/interface rating. Calculate complete loop resistance to the remote device, not one-way cable resistance. Include joints and branches according to the actual topology.
Cable capacitance, topology and device input capacitance can constrain a large or long segment even where static voltage appears acceptable. The wired M-Bus segment budget keeps unit-load, voltage and cable-capacitance margins independent so one healthy result cannot hide another exceeded limit.
Prove discovery and identity
Start with a known segment, then add meters in controlled groups. Resolve duplicate primary addresses before operational polling. Where secondary selection is used, retain the identification fields and wildcard behavior supported by the exact master and meters.
Do not identify a meter only by a location label. Preserve manufacturer/model and the address or secondary identity that was actually read. Replacing a meter should create an explicit identity/configuration change, not silently inherit the old record map.
Map telegram records exactly
For each required value, record whether it is instantaneous, cumulative, historical, tariff-specific or a status field. Preserve unit, multiplier, storage number, tariff number, sign and function. Heat meters may expose heating and cooling energy, volume, flow, flow and return temperatures, temperature difference, power, operating time and errors; exact availability varies by model and configuration.
A generic “energy” point is not enough. It may refer to the wrong tariff, direction or historical register while still looking plausible.
Keep the heat meter authoritative
For approved or billing meters, the meter's calculator, matched temperature pair, flow sensor and configured fluid model determine its declared energy result. Independent arithmetic can check scale and direction, but does not reproduce legal metrology.
The heat-meter sanity check uses a transparent single-phase sensible-heat approximation. Use it to find a unit, sign or order-of-magnitude error. Do not use it to replace the meter's approved energy register.
Failure and freshness tests
Disconnect one meter, short or overload only within an approved test arrangement, and interrupt the master connection. Verify that:
- unavailable meters are identified individually;
- stale values are not presented as fresh;
- retries do not starve healthy devices;
- cumulative totals do not reset or duplicate during recovery;
- timestamps and time zones are preserved; and
- alarms clear only when good data returns under the declared rule.
Handover record
Retain the as-built topology, cable data, unit-load and voltage budget, address schedule, meter inventory, record map, raw representative telegram, polling and retry settings, display reconciliation and failure-test results. Re-test after meter replacement, topology changes, master firmware or decoding changes.
Continue with the wired M-Bus application guide or open System Builder with the meter list and cable route.
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