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

RS-485 Installation and Set...
field) is received, each frequency converter or device
decodes it to determine which device is being addressed.
Modbus RTU messages addressed to zero are broadcast
messages. No response is permitted for broadcast
messages. A typical message frame is shown in Table 7.11.
Start Address
T1-T2-T3- 8 bits
T4
Table 7.11 Typical Modbus RTU Message Structure

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-
7 7
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 considers it a continuation of the previous message,
causing a time-out (no response from the follower), 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
follower device addresses are in the range of 0–247
decimal. The individual follower devices are assigned
addresses in the range of 1–247. (0 is reserved for
broadcast mode, which all slaves recognise.) A master
addresses a follower by placing the follower address in the
address field of the message. When the follower sends its
response, it places its own address in this address field to
let the master know which follower 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 follower. When a
message is sent from a master to a follower device, the
function code field tells the follower what action to
perform. When the follower 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 follower
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Design Guide
Function Data CRC
check
8 bits N x 8 16 bits
bits
Danfoss A/S © Rev. 06/2014 All rights reserved.
simply echoes the original function code. For an exception
response, the follower returns a code that is equivalent to
the original function code with its most significant bit set
to logic 1. In addition, the follower 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 chapter 7.8.9 Function Codes Supported by
End
Modbus RTU.
T1-T2-T3-

7.8.6 Data Field

T4
The data field is constructed using sets of 2 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 follower device
contains more information, which the follower 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 2 values are unequal, a bus time-out
results. The error-checking field contains a 16-bit binary
value implemented as 2 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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