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SMAX MODBUS®
6.2 Modbus frame description
The Modbus application protocol defines a simple Protocol Data Unit (PDU)
independent of the underlying communication layers:
Function code Data
MODBUS PDU
The mapping of Modbus protocol on a specific bus or network introduces some
additional fields on the Protocol Data Unit. The client that initiates a Modbus
transaction builds the Modbus PDU, and then adds fields in order to build the
appropriate communication PDU.
MODBUS SERIAL LINE PDU
Address field Function code Data CRC
MODBUS PDU
•
ADDRESS FIELD
: on Modbus Serial Line the address field only contains
the Slave address. The valid Slave node addresses are in the range of 0 –
247 decimal (see the
Node address [0004 hex]
register on page 65).
The individual Slave devices are assigned addresses in the range of 1 –
247. A Master addresses a Slave by placing the Slave address in the
ADDRESS FIELD of the message. When the Slave returns its response, it
places its own address in the response ADDRESS FIELD to let the Master
know which Slave is responding.
•
FUNCTION CODE
: the function code indicates to the Server what kind
of action to perform. The function code can be followed by a
DATA
field
that contains request and response parameters. For any further
information on the implemented function codes refer to the “6.4
Function codes” section on page 49.
•
DATA
: the
DATA
field of messages contains the bytes for additional
information and transmission specifications that the server uses to take
the action defined by the
FUNCTION CODE
. This can include items such
as discrete and register addresses, the quantity of items to be handled,
and the count of actual data bytes in the field. The structure of the
DATA
field depends on each
FUNCTION CODE
(refer to the “6.4
Function codes” section on page 49).
•
CRC (Cyclic Redundancy Check) : error checking field is the result of a
“Redundancy Checking” calculation that is performed on the message
contents. This is intended to check whether transmission has been
performed properly. The CRC field is two bytes, containing 16-bit binary
MAN SMAX MB E 1.2 Modbus® interface 45 of 84