The address field of a message frame contains 8 bits. Valid slave device addresses are in the range of 0 – 247 decimal. The individual slave devices are

assigned addresses in the range of 1 – 247. (0 is reserved for broadcast mode, which all slaves recognize.) 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 to let the master

know which slave is responding.

7.8.5 Function Field

The function field of a message frame contains 8 bits. Valid codes are in the range of 1-FF. Function fields are used to send messages between master

and slave. 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. 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 logic 1. In addition, the slave places a unique code into the data field of the

response message. This tells the master what kind of error occurred, or the reason for the exception. Please also refer to the sections

Supported by Modbus RTU

and

Exception Codes.

7.8.6 Data Field

The data field is constructed using sets of two hexadecimal digits, in the range of 00 to FF hexadecimal. These are made up of one RTU character. The

data field of messages sent from a master to slave device contains additional information which the slave must use to take the action defined by the

function code. This can include items such as coil or register addresses, the quantity of items to be handled, and the count of actual data bytes in the

field.

7.8.7 CRC Check Field

Messages include an error-checking field, operating on the basis of a Cyclical Redundancy Check (CRC) method. The CRC field checks the contents of

the entire message. It is applied regardless of any parity check method used for the individual characters of the message. The CRC value is calculated

by the transmitting device, which appends the CRC as the last field in the message. The receiving device recalculates a CRC during receipt of the message

and compares the calculated value to the actual value received in the CRC field. If the two values are unequal, a bus time-out results. The error-checking

field contains a 16-bit binary value implemented as two 8-bit bytes. When this is done, the low-order byte of the field is appended first, followed by the

high-order byte. The CRC high-order byte is the last byte sent in the message.

7.8.8 Coil Register Addressing

In Modbus, all data are organized in coils and holding registers. Coils hold a single bit, whereas holding registers hold a 2-byte word (i.e. 16 bits). All

data addresses in Modbus messages are referenced to zero. The first occurrence of a data item is addressed as item number zero. For example: The coil

known as 'coil 1' in a programmable controller is addressed as coil 0000 in the data address field of a Modbus message. Coil 127 decimal is addressed

as coil 007EHEX (126 decimal).

Holding register 40001 is addressed as register 0000 in the data address field of the message. The function code field already specifies a 'holding register'

operation. Therefore, the '4XXXX' reference is implicit. Holding register 40108 is addressed as register 006BHEX (107 decimal).

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