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User Manual
Modbus TCP Slave / Modbus Master
Document code: MN67507_ENG Revision 2.400 Page 33 of 45
OPERATION MODE:
R
OUTING
:
A few characteristics of the Modbus RTU package have been modified in the standard of the Modbus TCP protocol. Two bytes of the final CRC
were eliminated (no longer necessary for the information to reach its already corrected destination). The first byte of the slave identification
was changed, leaving the one that is called PDU. A frame denominated as MBAP header with dimensions of 7 bytes was added to the head of
the PDU.
It is composed by the following:
Word transaction identifier (recopied from the slave in the response phase);
Word protocol identifier (0=Modbus protocol);
Word length (number of successive bytes);
Byte unit identifier (used for the routing operation).
By using the last byte of the MBAP header, it is possible to carry out the routing from a requested Modbus TCP toward a serial line using the
address from the slave which is specified by the byte unit identifier.
Example:
A requested Modbus TCP made in the device as the address 192.168.0.200 for the holding register address $2000, which is MBAP unit
identifier has the value of 2. It will be followed by the request on the serial for the device with the address 2 at the word $2000.
After the request is made, the RTU will respond. The master TCP will be given the same response which will be reconstructed according to
the specifications of the Modbus TCP.
If the RTU slave responds with an exception, that exception code will be transmitted again to the TCP master. If the RTU slave does not
respond in the estimated time defined by the Timeout parameter, an exception response will be given: error code $0B.
N
ATTING
:
When NAT operation mode is used, the requested data will be processed through a table of translations of the Modbus network addresses.
