Industrial protocols

Wired M-Bus segment budget calculator

Check a wired M-Bus segment against the master's unit-load capacity, the voltage at the farthest meter and the OMS capacitance guidance for the baud rate.

Calculate

Segment load

Use the stated UL rating for each connected device and the manufacturer’s master capacity.

devices
UL/device
Use the device datasheet. Current OMS identification covers 1–4 UL; larger legacy loads are flagged for review.
UL
Adds current capacity only. Enter future cable and devices explicitly when checking capacitance.
UL
Use the maximum connected unit loads stated by the master manufacturer.
Master and farthest current path

Treat the resistance as the complete effective current path to the farthest device, including both conductors and any specified source or protection resistance.

V DC
Ω
Total bus capacitance

Use total installed cable length across all branches. Cable and device capacitance must come from current datasheets.

km
nF/km
nF/device
Current OMS TR02 cites a 0.5 nF slave-terminal limit; use the actual device value when known.

Worked example

Segment budget status

Within entered limits 

Design load / entered master capacity
40 / 60 UL
Master capacity margin
20 UL
Design idle current
60 mA
Far-end mark voltage, conservative estimate
31.56 V DC
Margin above the 24 V guide target
7.56 V
Estimated total segment capacitance
115 nF
Capacitance margin at 2400 baud
285 nF
Specified slave response-current increase
11–20 mA

The entered load, conservative voltage and capacitance budgets are within the cited planning limits. This is not installation approval; verify the exact hardware, topology and measured bus values.

Method and assumptions

Formula

ULdesign = N × ULdevice + ULreserve; Iidle = ULdesign × 1.5 mA; Ufar = Umark − IidleRsegment; Ctotal = LtotalCcable + NCdevice

The voltage calculation conservatively places the complete design current at the farthest effective segment resistance. The capacitance calculation uses total installed cable length, not only the farthest branch.

Limits of this result

  • Enter the master capacity and device unit loads from their current manufacturer data; a device count alone is not a load budget.
  • The far-end voltage result is a conservative lumped-current estimate using the entered effective current-path resistance. It is not a distributed-network simulation.
  • The capacitance budget adds the entered cable and connected-device capacitance. Reserve unit loads do not add future cable or device capacitance.
  • The estimate does not verify waveform shape, rise/fall time, topology, EMC, isolation, inrush, collision handling or polling reliability.
  • Do not use the result as installation approval. Confirm the exact master, every meter or coupler, cable construction, topology and measured commissioning values.

Technical sources

  1. Technical Report 02 — Wired M-BusOpen Metering System Group · Version 2.0.3 · 2023-03-29
  2. Technical Report 09 — Guide for Wired M-Bus InstallationsOpen Metering System Group · Version 1.0.3 · 2022-05-27

Put the value to work

Read the related engineering guidance, or carry this task into System Builder to identify the signal path and EpiSensor products.

Continue in System Builder Read: What Is SCADA?

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