Hach ANATEL PAT700 User Manual - 11.2 Modbus/tcp Driver

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Modbus Protocol

11.2 Modbus/TCP driver

11.2.1 Modbus protocol

A typical Modbus/TCP frame consists of the following fields:

The MBAP Header (Modbus Application Protocol Header) consists of 7 bytes of 

information:

The Function Code field of a message contains 8 bits. Valid function codes are in the 

range of 1 - 255 decimal. The function code instructs the slave what kind of action to 

take. The Modbus/TCP feature only supports Function Codes: FC3 (Read Multiple 

Registers) and FC6 (Preset Single Register). The 125-register limitation is established for 

the Modbus/TCP standard to maintain consistency with original Modbus protocol 

standard, even though a TCP/IP packet can support more data.

When a slave responds to the master, it uses the function code field to indicate either a 

normal response or that some type of error has occurred. For a normal response, the 

slave echoes the original function code. In an error condition, the slave echoes the 

original function code with its MSB set to logic 1.

The Data field is constructed using sets of two hexadecimal digits in the range of 00 to 

FF. According to the network's serial transmission mode, these digits can be made of a 

pair of ASCII characters or from one RTU character. The data field also contains 

additional information that the slave uses to execute the action defined by the function 

code. This can include internal addresses, quantity of items to be handled, etc.

The data field of a response from a slave to a master contains the data requested if no 

error occurs. If an error occurs, the field contains an exception code that the master uses 

to determine the next action to be taken. The data field can be nonexistent in certain 

types of messages.

MBAP Header

Function

Data

Transaction Identifier

2 bytes

It is used for transaction pairing i.e. identification of 

Request/Response transaction. The Modbus/TCP server copies 

the transaction identifier of the request in to the response.

Protocol Identifier

2 bytes

It is used for intra-system multiplexing. The Modbus/TCP 

protocol is identified by the value 0.

Length

2 bytes

The length field is a byte count of the following fields, including 

the Unit Identifier.

Unit Identifier

1 byte

This field is used for intra-system routing purpose. It is typically 

used to communicate to a MODBUS or a MODBUS+ serial line 

slave through a gateway between an Ethernet TCP/IP network 

and a MODBUS serial line. This field is set by the MODBUS 

Client in the request and must be returned with the same value 

in the response by the server.