35
The Modbus Message frame is shown below
DEVICE FUNCTION DATA CRC
ADDRESS CODE CHECK
8 BITS 8 BITS k x 8 BITS 16 BITS
nn nn nnn... nnnn
where: n – character, k – integers depend on the contents of the data format.
6.3 Device Address
The address message frame contains eight bits. The slave device addresses are in the
range of 1 ... 199 decimal. A master addresses a slave by placing the slave address in
the address field of the message. When the slave sends its response, it places its own
address in this address field of the response to let the master know which slave is
responding. Address 0 is used for the write command broadcast that commands all
devices on network, which all slave devices recognize.
6.4 Function Code
The function code field of a message frame contains eight bits (RTU). Valid codes are in
the range of 1 ... 255 decimal. Of these, some codes are applicable for i-Series
controllers. When a message is sent from a master to a slave device the function code
field tells the slave what kind of action to perform.
The following functions are supported by i-Series devices:
Table 6.1 Function Code
Function Code Function Description
03 Read holding register Reads the binary contents of holding
registers in the slave
04 Read input register Reads the binary contents of input register
in the slave.
06 Preset (Write to) Preset (Write) a value into single holding
single register register
08 Diagnostic Series of tests for checking communication
between master and slave
When the slave responds to the master, it uses the function code field to indicate either a
normal (error-free) response or that some kind of error occurred (called an exception
response). For a normal response, the slave simply echoes the original function code.
For an exception response, the slave returns a code that is equivalent to the original
function code with its most significant bit set to a logic 1.
6.5 Data Field
The data field is constructed using sets of two hexadecimal digits, in the range of 00 to
FF hexadecimal. The data field of messages sent from a master to slave devices
contains additional information, which the slave must use to take the action defined by
the function code. This can include items like discrete and register addresses, the
quantity of items to be handled, and the count of actual data bytes in the field.
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