Modbus Communication Driver

Modbus Communication Driver is intended for data connection between Reliance's runtime software and devices communicating via the Modbus RTU and/or Modbus TCP communication protocols. The driver implements basic communication functions for reading and writing data in both the Client mode and the Server mode.

The implemented communication functions in the Client mode

01 Read Coil Status – reading digital outputs

02 Read Input Status – reading digital inputs

03 Read Holding Registers – reading holding registers

04 Read Input Registers – reading input registers

05 Force Single Coil – writing a single digital output

06 Preset Single Register – writing a single holding register

15 Force Multiple Coils – writing multiple digital outputs

16 Preset Multiple Registers – writing multiple holding registers

20 Read General Reference – reading the extended memory

22 Mask Write 4X Register – mask writing a single holding register

The implemented communication functions in the Server mode

01 Read Coil Status – reading digital outputs

02 Read Input Status – reading digital inputs

03 Read Holding Registers – reading holding registers

04 Read Input Registers – reading input registers

05 Force Single Coil – writing a single digital output

06 Preset Single Register – writing a single holding register

16 Preset Multiple Registers – writing multiple holding registers

If any of the writing functions is not supported by the device, this function can then be deactivated.

Communication with devices runs in one of the following ways:

• RS-232/485 serial link
• Ethernet (TCP/IP or UDP)

The desired channel type can be selected in the Reliance development environment's Project Structure Manager (device connection).

A Modicon M580 programmable logic controller

Starting up the driver

The communication driver is a DLL loaded into memory by the Reliance 4 Driver Server program, which can be launched either as a Windows service at Windows startup or as a standard application at the start of a visualization project. Information required to make a connection with devices is read from the visualization project. After reading this information, communication to all connected devices is activated.

Addressing tags

Modicon systems' memory is divided into the following four basic areas: discrete inputs, discrete outputs, input registers, holding registers. In Modicon systems, these areas are addressed absolutely (see the "Modicon addressing" column in the table below). In the Reliance system, however, tags are addressed relatively always starting at zero (see the "Reliance addressing" column in the table below).

Example

In the Reliance system, a tag at 40011 will be addressed as follows:
Register type: Holding registers; Address: 10

Reading data

Data is read periodically based on the defined communication zones. The tag value and the data of the communication zone in which the tag is located are always updated together. The tag data type must match the size of a data point in the selected register type. The size of data points is defined by the Modbus protocol (Coils, Inputs – bool; Holding registers, Input registers – word).

Detecting the loss of connection with the device

The loss of connection is detected if the communication driver doesn’t receive a valid response Nx consecutively (N represents the maximum number of failed communications). This parameter can be adjusted.

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