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Working with Modbus TCP Devices

Introduction

The filters for Modbus over TCP communication are located in the System :: Modbus TCP

Establishing Connection

To communicate with a ModbusTCP device, first, a connection needs to be established. The ModbusTCP_Connect filter should be used for establishing the connection. The default port for ModbusTCP protocol is 502. Usually, a connection is created before the application enters its main loop and it remains valid through multiple iterations of the process. It becomes invalid when it is explicitly closed using ModbusTCP_Close filter or when an I/O error occurs.

The Modbus filters work on the same socket as TCP IP filters. Therefore, it might happen that in case of the connection issues, information about TCP IP errors will appear.

If Modbus device supports Keep-Alive mechanism it is recommended to enable it to increase chance of detecting a broken connection. For more details, please read Using TCP/IP Communication.

Reading and Writing Values

FabImage Studio provides set of filters to manage Modbus objects. The following is table of object types provided by Modbus:

Object Type	Access	Size
Coil	Read-Write	1-bit
Discrete Input	Read-Only	1-bit
Input Register	Read-Only	16-bits
Holding Register	Read-Write	16-bits

The Coils, Inputs and Registers in ModbusTCP protocol are addressed starting at zero. The table below lists filters to control object types mentioned above.

Object Type	Read	Write
Coil	ModbusTCP_ReadCoils	ModbusTCP_WriteCoil ModbusTCP_ForceMultipleCoils
Discrete Input	ModbusTCP_ReadDiscreteInputs	Read-Only
Input Register	ModbusTCP_ReadInputRegisters_AsByteBuffer ModbusTCP_ReadInputIntegerRegisters ModbusTCP_ReadInputRealRegisters	Read-Only
Holding Register	ModbusTCP_ReadMultipleRegisters_AsByteBuffer ModbusTCP_ReadMultipleIntegerRegisters ModbusTCP_ReadMultipleRealRegisters	ModbusTCP_WriteSingleRegister ModbusTCP_WriteMultipleRegisters_AsByteBuffer ModbusTCP_WriteMultipleIntegerRegisters ModbusTCP_WriteMultipleRealRegisters

Sample Application

The following example illustrates how to use ModbusTCP filters.

Main macrofilter - establish and close connection

Main Loop - communication with Modbus device

Inspection - sample inspection algorithm

Custom Functions

To support device-specific functions, FabImage Studio provides ModbusTCP_SendBuffer filter. This filter allows you to create a custom Modbus frame.

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