Modbus Rtu Message Structure; Start/Stop Field; Address Field; Function Field - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

RS-485 Installation and Set...

7.8.2 Modbus RTU Message Structure

The transmitting device places a Modbus RTU message
into a frame with a known beginning and ending point.
This allows receiving devices to begin at the start of the
message, read the address portion, determine which
device is addressed (or all devices, if the message is
broadcast), and to recognize when the message is
completed. Partial messages are detected, and errors are
set as a result. Characters for transmission must be in
hexadecimal 00 to FF format in each field. The adjustable
frequency drive continuously monitors the network bus,
also during 'silent' intervals. When the first field (the
address field) is received, each adjustable frequency drive
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 7.13.
Start Address Function
T1-T2-T3- 8 bits 8 bits
T4
Table 7.13 Typical Modbus RTU Message Structure

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 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.
This causes a timeout (no response from the follower),
since the value in the final CRC field is not valid for the
combined messages.
MG18C522
Design Guide
Data CRC End
check
N x 8 16 bits T1-T2-T3-
bits T4
Danfoss A/S © Rev. 2014-01-14 All rights reserved.

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 followers recognize.) 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 kind of 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 some kind of error occurred (called
an exception response). For a normal response, the
follower 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 tells the master what kind of error occurred, or the
reason for the exception. Also refer to
chapter 7.8.10 Function Codes Supported by Modbus RTU and
chapter 7.8.11 Modbus 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 follower device contains additional
information which the follower 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.
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