7.2 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. As previously stated (see the “4.4.1 Setting the node
address: Node ID (Figure 7)“ section on page 21), the valid Slave node
addresses are in the range of 0 – 247 decimal. 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 “7.4
Function codes” section on page 53.
•
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 “7.4
Function codes” section on page 53).
•
CRC (Cyclic Redundancy Check)
: error checking field is the result of a
“Redundancy Check” 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 RD6 MB E 1.1
Modbus® interface
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RD6 Modbus®
