Start/Stop Field; Address Field; Function Field; Data Field - Danfoss VLT DriveMotor FCP 106 Programming Manual

RS485 Installation and Set-...
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 0 are broadcast
messages. No response is permitted for broadcast
messages. A typical message frame is shown in Table 3.14.
Start Address Function
T1-T2-T3- 8 bits 8 bits
T4
Table 3.14 Typical Modbus RTU Message Structure

3.8.3 Start/Stop Field

Messages start with a silent period of at least 3.5 character
intervals. The silent period 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 behavior causes a timeout (no response from the
slave), since the value in the final CRC field is not valid for
the combined messages.

3.8.4 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 recognize.) 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.

3.8.5 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 kind of action to
MG03N202
Programming Guide
Data CRC End
check
N x 8 bits 16 bits T1-T2-T3-
T4
Danfoss A/S © 11/2015 All rights reserved.
perform. When the slave 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 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 kind of error
occurred, or the reason for the exception. Also refer to
chapter 3.8.12 Function Codes Supported by Modbus RTU and
chapter 3.8.13 Modbus Exception Codes.

3.8.6 Data Field

The data field is constructed using sets of 2 hexadecimal
digits, in the range of 00 to FF hexadecimal. These digits
are made up of 1 RTU character. The data field of
messages sent from a master to a slave device contains
additional information which the slave must use toact
according to the function code. The information 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.

3.8.7 CRC Check Field

Messages include an error-checking field, operating based
on a cyclic 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 timeout
results. The error-checking field contains a 16-bit binary
value implemented as 2 8-bit bytes. After the implemen-
tation, 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.

3.8.8 Coil Register Addressing

For coil register addressing, refer to Modbus RTU Operating
Instructions.

3.8.9 Access via PCD Write/Read

The advantage of using the PCD write/read configuration is
that the controller can write or read more data in one
telegram. Up to 63 registers can be read or written to via
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