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.

The implemented communication functions:

• 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 system 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 Regs – writing multiple holding registers
• 22 Mask Write 4X Register – mask writing a single holding register

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)

Connection type can be selected in the Reliance development environment in the Project Structure Manager (connecting a device).

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 the devices is read from the visualization project. After reading this information, communication to the devices is activated.

Addressing tags

Memory of the Modicon systems is divided into the four basic areas: digital inputs, digital outputs, input registers, holding registers. In the Modicon systems, these areas are addressed absolutely, see the table below, the “Modicon addressing” column. In the Reliance system, however, the tags are addressed relatively starting always at zero, see the table below, the “Reliance addressing” column.

Example

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

Reading data

Reading the data runs 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 register – 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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