Start/Stop Field; Address Field; Function Field; Data Field - Danfoss VLT AQUA Drive FC 202 Design Manual

RS-485 Installation and Set...

7.8.3 Start/Stop Field

Messages start with a silent period of at least 3.5 character
intervals, implemented as a multiple of character intervals
at the selected network baud rate (shown as Start T1-T2-
T3-T4). The first transmitted field 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 is the address field of a new
message. Similarly, if a new message begins before 3.5
character intervals after a previous message, the receiving
device will consider it a continuation of the previous
message, causing a time-out (no response from the slave),
since the value in the final CRC field is not 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 recognise.) 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 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 an error has 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 code tells the
master what error occurred, or the reason for the
exception. See 7.8.9 Function Codes Supported by Modbus
RTU.
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VLT AQUA Drive FC 202 Design Guide

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
sequences are made up of one RTU character. The data
field of messages sent from a master to slave device
contains more information, which the slave must use to do
what is defined by the function code. This information can
include items such as coil or register addresses, the
quantity of items, and the count of actual data bytes in
the field.

7.8.7 CRC Check Field

Messages include an error-checking field, operating based
on 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 transmitting
device calculates the CRC value then 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. After error-
checking, 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.
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