M2MB Session Master
The M2MB Session Master connects a standard Modbus RTU master device — such as a PLC, HMI, SCADA interface, service laptop, or data logger — to the M2MB multi-master backbone.
It does not replace Modbus RTU. Instead, it adds a lightweight session header to the existing Modbus data stream, allowing many independent RTU masters to communicate through the same shared twisted-pair backbone.
This makes it possible to expand proven Modbus systems without forcing a change of PLC, HMI, drives, sensors, or existing RTU devices.
Typical examples include a local HMI, remote HMI, data logger, SCADA system, service laptop, or secondary controller that all require live access to the same Modbus RTU network.
What the Session Master does
A conventional Modbus RTU master sends normal RTU frames to the Session Master. The Session Master wraps each frame with a small session identifier before placing it on the M2MB backbone. This session identifier tells the system where the request came from, so that the reply can be routed back to the correct master.
Receives standard RTU
Connects locally to a normal Modbus RTU master using that master device's configured baudrate.
Adds session identity
Applies a lightweight session header without changing the Modbus payload itself.
Joins the backbone
Transports the request over the high-speed M2MB backbone to the required Session Slave / Gateway.
Flexible baudrate structure
Each Session Master can use a different local Modbus RTU baudrate while the M2MB backbone runs at its own high-speed baudrate. This allows old and new devices to coexist without forcing the entire system to run at the slowest local link speed.
- Example: Master 1 at 9600 baud, Master 2 at 19.2 kbit/s, Master 3 at 115.2 kbit/s
- All three can connect through individual Session Masters to a 500 kbit/s M2MB backbone
- Local master configuration remains familiar and Modbus RTU compatible
Backbone performance
The M2MB backbone currently operates at speeds up to 500 kbit/s. Because the backbone avoids much of the idle-time overhead of conventional RTU polling, the effective system performance can be comparable to many separate RTU links.
In practical system terms, one 500 kbit/s M2MB backbone can provide throughput similar to 10–20 independent Modbus RTU masters operating at 115.2 kbit/s, each on its own cable.
Deterministic arbitration
M2MB uses a field-proven bus arbitration system to avoid packet loss when two devices attempt to start transmitting at the same time. Simultaneous starts are resolved on the bus, with the losing node withdrawing without corrupting the winning packet.
This is what allows M2MB to behave as a practical multi-master Modbus transport rather than a simple shared wire with collision risk.
Many existing “multi-master” Modbus solutions operate by multiplexing or scheduling communication between masters, effectively allowing only one active master at a time.
M2MB takes a different approach by allowing independent masters to communicate dynamically over the same backbone using deterministic arbitration and session-aware routing.
In many ways, M2MB represents a natural evolution of Modbus RTU toward true multi-master industrial communication.
Typical applications
- Adding a second HMI to a machine without duplicating the slave network
- Providing a remote HMI while keeping a local operator panel active
- Connecting a service laptop without interrupting normal PLC/HMI communication
- Adding a data logger or SCADA system to an existing Modbus RTU installation
- Allowing several controllers to access live device registers without shadow-register gateways
Beyond tablet counting equipment
XIAC Australia's main product line is related to tablet and capsule counting equipment, but M2MB was specifically designed for standard industrial Modbus RTU systems using twisted pair cable.
The same requirement appears in many industries: reliable Modbus devices are already installed, but more than one master needs access. M2MB is therefore just as valid for tablet counting equipment as it is for other Modbus RTU networks.