Danfoss VLT HVAC Drive FC 100 Series, Руководство по конструкции

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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 Data CRC
check
End
T1-T2-T3-
T4
8 bits 8 bits N x 8
bits
16 bits T1-T2-T3-
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 will be the address field of a
new message. Similarly, if a new message begins prior to
3.5 character intervals after a previous message, the
receiving device will consider it a continuation of the
previous message. This will cause a timeout (no response
from the slave), since the value in the final CRC field will
not be valid for the combined messages.
7.8.4 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.
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 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
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 tells the master
what kind of error occurred, or the reason for the
exception. Please also refer to
7.8.10 Function Codes
Supported by Modbus RTU and 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 slave device contains additional
information that the slave 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.
7.8.7 CRC Check Field
Messages include an error-checking field, operating on the
basis of 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 two values are unequal, a bus
timeout results. The error-checking field contains a 16-bit
binary value implemented as two 8-bit bytes. When this is
done, the low-order byte of the field is appended first,
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