Modbus/Tcp Driver; Modbus Protocol - Hach Anatel PAT700 User Manual

Modbus Protocol

11.2 Modbus/TCP driver

11.2.1 Modbus protocol
Page 146
A typical Modbus/TCP frame consists of the following fields:
MBAP Header
The MBAP Header (Modbus Application Protocol Header) consists of 7 bytes of
information:
Transaction Identifier 2 bytes
Protocol Identifier 2 bytes
Length 2 bytes
Unit Identifier 1 byte
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.
Function
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.
It is used for intra-system multiplexing. The Modbus/TCP
protocol is identified by the value 0.
The length field is a byte count of the following fields, including
the Unit Identifier.
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.
Data