Modbus Protocol is a messaging structure developed by Modicon in 1979, used to establish master-slave/client-server communication between intelligent devices. Modbus is a de facto standard, truly open and the most widely used network protocol in the industrial manufacturing environment. Modbus is literally implemented by hundreds of vendors on thousands of different devices in order to transfer discrete/analog I/O and register data between control devices using Modbus. Modbus continues to enable millions of automation devices to communicate.
There are different flavors of Modbus. The most common is Modbus RTU which is based on serial (twisted pair communication) like RS485 and RS232. There is also Modbus ASCII. Today Modbus over Ethernet is growing significantly. It is called Modbus TCP and is basically the Modbus RTU protocol embedded into TCIP packets. It is an excellent choice to connect Modbus devices to SCADA systems or PLC’s.
Modbus is an application layer messaging protocol, positioned at level 7 of the OSI model that provides client/server communication between devices connected on different types of buses or networks.
Modbus is a request/reply protocol and offers services specified by function codes. Modbus function codes are elements of Modbus request/reply PDUs (Protocol Data Unit).

The Modbus application data unit is built by the client that initiates a Modbus transaction. The function indicates to the server what kind of action to perform. The Modbus application protocol establishes the format of a request initiated by a client. The function code field of a Modbus data unit is coded in one byte. Valid codes are in the range of 1 ... 255 decimal (128 – 255 reserved for exception responses). When a message is sent from a Client to a Server device the function code field tells the server what kind of action to perform.

Sub-function codes are added to some function codes to define multiple actions. For example a client can read the ON / OFF states of a group of discrete outputs or inputs or it can read/write the data contents of a group of Modbus registers. When the server responds to the client, it uses the function code field to indicate either a normal (error-free) response or that some kind of error occurred (called an exception response). For a normal response, the server simply echoes to the request the original function code.

Modbus bases its data model on a series of tables that have distinguishing characteristics.
IntelliCom has several gateways and network interfaces based on Modbus. Please contact IntelliCom for further information about our standard products or if you would like us to develop your Modbus Gateway.

• Network Size: Depends on performance in the device and network. Typically about 250 Modbus devices can be used in a network.
• Network Length: Depends on baud rate. For example a baudrate of 9.6 kbit/s
• Baud Rate: Based on RS485 limitations, for example up to 115 kbit/s
• Bus Topology: Bus

Features
• Connects Modbus RTU products to SCADA or PLC’s over Ethernet
• Supports Ethernet, Internet, LAN, GSM/GPRS, Modbus TCP, email, FTP etc
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Features
• Connects Modbus RTU products to SCADA or PLC’s over Ethernet
• Transparent Gateway between Modbus RTU and Modbus TCP
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Features
• Connects Modbus RTU/ASCII products to any Windows based software
• Transparent connection
• 2 ports (1 RS485 + 1 RS232)
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Features
• Modbus I/O: 4 DI, 4 DO, 2AI, 1AO, “ RTD (Temperature)
• Can be accessed through Internet and Mobile phones
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