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Modbus RTU Protocol

The DMB devices implement the Modbus RTU Protocol. The Modbus protocol is a single master protocol. Therefore a slave only sends answer messages to former requests of the Modbus master device. The Modbus RTU standard defines a binary communication inside the Modbus Frame.


A Modbus RTU message frame consists of the following parts:

Modbus Frame

The 4 fields have the following meaning:

  • 1. Address Field: States the address of the slave that is addressed in the actual communication. The master itself has no address. The address 0 is reserved for broadcast communication. The addresses 248 to 255 are actually reserved by the Modbus standard.
  • 2. Function Code: A 1 Byte field that contains the command the slave has to process. The function codes are standardized for all Modbus devices. The DMB devices implement only a limited subset of function codes, depending on the devices.
  • 3. Data: This field contents the information referring to the function code (e.g. the address of a requested register). The data needed for each function code can be found in the specifications.
  • 4. CRC: The last field contains the two bytes of the CRC used to verify the data frame.


A start or an end of a Modbus RTU frame is indicated by a 3.5 Byte long pause which means that no devices on the network sends data.

RS485 Network for Modbus

The very common form of a Modbus network based on 2-wire RS485 is described detailed in "MODBUS over Serial Line - Specification and Implementation Guide V1.02". The RS485 is defined in the EIA-485 (meanwhile TIA-485) standard.

Below the only basic structure is described:


According to the Modbus Organization the devices are connected via 2 data lines and a third functional line:

  • D0   (which is referred to as “B” in the EIA/TIA-485)
  • D1   (which is referred to as “A” in EIA/TIA-485)
  • Common   (which is referred to as “C” in EIA/TIA-485)


The original RS485 standard allows 32 devices each with 1 so-called unit load. Modern devices offer a unit load which is only a fraction of 1 to enable more devices on the bus. The DMB devices have a unit load of 1/8 unit load (Rin ≥ 96 kΩ) to enable up to 256 devices in a RS485 network without the need for a repeater. This number is actually limited by the Modbus address space to 247.

The Resistors RT are used to minimize the reflections compared to an open ended line. The optimum value of the resistor depends on the wave impedance of the cable used. However a value of 120 Ω is a common choice.

The Polarization Network is needed to ensure proper potentials when none of the devices are sending and thus the lines D1 and D0 are floating. The value of RP depends on several things like bus load or termination resistors. The Modbus Organization suggests values from 450 Ω to 650 Ω for RP. The use of a polarization Network is strongly suggested to obtain a robust stable network. The polarization resistors are usually integrated in the master device.

Devices of the DRAGO DMB series don’t have internal resistors for termination or polarization.

Connection via In-Rail-Bus

The primary way to connect the DMB devices is the rear In-Rail-Bus connector (A-E). Third-party devices without the In-Rail-Connector can be connected by an In-Rail-Bus Power-Terminals (order-no.: DZU 1401; DZU 1402). Some DMB devices also internally connect the Modbus signals to the terminals 5, 6 and 8. An overview of all connections on the enclosure is shown below:

Connections of DRAGO DMB devices

RS485 (Modbus) signal name In-Rail-Bus connector Optional Modbus terminals
D1 (A) A 5
D0 (B) B 6


Example: PC with USB to RS485 converter

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