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iSMA-B-AAC20 M-Bus User Manual


This manual contains information about an M-Bus protocol in the iSMA-B-AAC20 controllers. M-Bus devices can be connected directly only to the iSMA-B-AAC20-M hardware version with the M-Bus interface (max. 20 devices) and with firmware version 5.1 and above. The M-Bus devices can also be connected to all iSMA-B-AAC20 controllers with firmware 5.1 and above using M-Bus-IP gateway.

AAC20-LCD controller

Revision History





22 Nov 2016

First edition


20 Apr 2017

Added action “Reset Stat” in the network components


27 Jan 2020

Replaced environment of programming from Workplace to iSMA Tool


28 Feb 2022


Revision history

About M-Bus

The M-Bus (Meter Bus) was developed to fill the need for a system for the networking and remote reading of utility meters, for example, to measure the consumption of gas or water in the house. This bus fulfils the special requirements of remotely powered or battery-driven systems, including consumer utility meters. When interrogated, the meters deliver the data they have collected to a common master, for example, a DDC controller or a hand-held computer, connected at periodic intervals to read all utility meters of a building.

M-Bus Topology and Cable

The M-Bus is a hierarchical system, with communication controlled by a master (for example the iSMA-B-AAC20-M). The M-Bus consists of the master, a number of slaves (end-equipment meters), and a two-wire connecting cable. The slaves are connected in parallel to the transmission medium - the connecting cable.

The M-Bus network

A two-wire cable (jYStY N*2*0.8 mm) is used as the transmission medium for the M-Bus. The maximum distance between the slave and the repeater is 350 m; this length corresponds to a cable resistance of up to 29 Ω. This distance applies for the standard configuration having a baud rate between 300 and 9600 baud rate, and a maximum of 250 slaves. The maximum distance can be increased by limiting the baud rate and using fewer slaves, but the bus voltage in the Space state must at no point in a segment fall below 12 V, because of the remote powering of the slaves. In the standard configuration, the total cable length should not exceed 1000 m, in order to meet the requirement of a maximum cable capacitance of 180 nF.

M-Bus Addressing

The M-Bus devices are using two types of addressing:

  • primary: this address is assigned by the user in a commissioning process (all new M-Bus devices have this address, set by the factory to 0); this type of address has a limited range from 0 to 250;
  • secondary: this address has a wider range than primary and by default contains a device serial number. All out of box devices connected to the bus have unique secondary addresses.
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