Start/Stop Field; Address Field; Function Field; Data Field - Danfoss VLT AutomationDrive FC 301 Design Manual

RS-485 Installation and Set...

12.11.3 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 time-out (no response from the follower),
since the value in the final CRC field is not valid for the
combined messages.

12.11.4 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 followers 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.
12 12

12.11.5 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 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 12.11.10 Function Codes Supported by Modbus RTU
and chapter 12.11.11 Modbus Exception Codes
184
®
VLT AutomationDrive FC 301/FC 302 Design Guide, 0.25-75 kW

12.11.6 Data Field

The data field is constructed using sets of 2 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.

12.11.7 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 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 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. 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.

12.11.8 Coil Register Addressing

In Modbus, all data are organised in coils and holding
registers. Coils hold a single bit, whereas holding registers
hold a 2-byte word (i.e. 16 bits). All data addresses in
Modbus messages are referenced to zero. The first
occurrence of a data item is addressed as item number
zero. For example: The coil known as 'coil 1' in a
programmable controller is addressed as coil 0000 in the
data address field of a Modbus message. Coil 127 decimal
is addressed as coil 007EHEX (126 decimal).
Holding register 40001 is addressed as register 0000 in the
data address field of the message. The function code field
already specifies a 'holding register' operation. Therefore,
the '4XXXX' reference is implicit. Holding register 40108 is
addressed as register 006BHEX (107 decimal).
MG33BF02 - Rev. 2013-12-20