Modbus Rtu Message Framing Structure; Frequency Converter With Modbus Rtu; Modbus Rtu Message Structure; Start/Stop Field - Danfoss VLT AutomationDrive FC 300 Design Manual

RS-485 Installation and Set...
10.11 Modbus RTU Message Framing
Structure
10.11.1 Frequency Converter with Modbus
RTU
The controllers are set up to communicate on the Modbus
network using RTU (Remote Terminal Unit) mode, with
each byte in a message containing 2 4-bit hexadecimal
characters. The format for each byte is shown in
Table 10.10.
Start
bit
Table 10.9 Example Format
Coding System
Bits Per Byte
10 10
Error Check Field
Table 10.10 Bit Detail

10.11.2 Modbus RTU Message Structure

The transmitting device places a Modbus RTU message
into a frame with a known beginning and ending point.
Receiving devices are able 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 recognise when the message is
completed. Partial messages are detected and errors set as
a result. Characters for transmission must be in
hexadecimal 00 to FF format in each field. The frequency
converter continuously monitors the network bus, also
during 'silent' intervals. When the first field (the address
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 10.12.
Start Address
T1-T2-T3- 8 bits
T4
Table 10.11 Typical Modbus RTU Message Structure
248
®
VLT
Data byte Stop/
Parity
8-bit binary, hexadecimal 0–9, A-F. two
hexadecimal characters contained in each 8-
bit field of the message
1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity; no bit for no
parity
1 stop bit if parity is used; 2 bits if no parity
Cyclical Redundancy Check (CRC)
Function Data CRC
check
8 bits N x 8 16 bits
bits
AutomationDrive FC 300 Design Guide 90-1200 kW

10.11.3 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
Stop
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.

10.11.4 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.

10.11.5 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
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 10.11.10 Function Codes Supported by
End
Modbus RTU.
T1-T2-T3-
T4
MG34S202 - Rev. 2013-08-19