7.8.3. Start / Stop Field
Messages start with a silent period of at least 3.5 character intervals. This is implemented as a
multiple of character intervals at the selected network baud rate (shown as Start T1-T2-T3-T4).
The first field to be transmitted is the device address. Following the last transmitted character, a
similar period of at least 3.5 character intervals marks the end of the message. A new message
can begin after this period. The entire message frame must be transmitted as a continuous stream.
If a silent period of more than 1.5 character intervals occurs before completion of the frame, the
receiving device flushes the incomplete message and assumes that the next byte will be the ad-
dress field of a new message. Similarly, if a new message begins prior to 3.5 character intervals
after a previous message, the receiving device will consider it a continuation of the previous mes-
sage. This will cause a time-out (no response from the slave), since the value in the final CRC field
will not be valid for the combined messages.
7.8.4. Address Field
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
Function Codes 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 hexa-
decimal. 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 calcu-
lated 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. RS-485 Installation and Set-up
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®
HVAC Drive Design Guide
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