Start/Stop Field; Address Field; Function Field; Data Field - Danfoss VLT Compressor Drive CDS 803 Design Manual

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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 telegrams addressed to 0 are broadcast
telegrams. No response is permitted for broadcast
telegrams. A typical telegram frame is shown in Table 7.14.
Start Address
T1-T2-T3- 8 bits
T4
Table 7.14 Typical Modbus RTU Telegram Structure

7.8.3 Start/Stop Field

Telegrams start with a silent period of at least 3.5 character
7 7 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 telegram. A new
telegram can begin after this period.
Transmit the entire telegram frame 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 telegram and assumes that
the next byte is the address field of a new telegram.
Similarly, if a new telegram begins before 3.5 character
intervals after a previous telegram, the receiving device
considers it a continuation of the previous telegram. This
behavior causes a timeout (no response from the slave),
since the value in the final CRC field is not valid for the
combined telegrams.

7.8.4 Address Field

The address field of a telegram 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 telegram.
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 telegram frame contains 8 bits. Valid
codes are in the range of 1–FF. Function fields are used to
send telegrams between master and slave. When a
telegram is sent from a master to a slave device, the
function code field tells the slave what kind of action to
58
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VLT Compressor Drive CDS 803
Function Data CRC
check
8 bits N x 8 bits 16 bits
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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
End
its most significant bit set to logic 1. In addition, the slave
places a unique code into the data field of the response
T1-T2-T3-
telegram. This code tells the master what kind of error
T4
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 2 hexadecimal
digits, in the range of 00–FF hexadecimal. These digits are
made up of 1 RTU character. The data field of telegrams
sent from a master to a slave device contains additional
information which the slave must use to perform
accordingly. The information can include items such as:
•
Coil or register addresses.
•
The quantity of items to be handled.
•
The count of actual data bytes in the field.

7.8.7 CRC Check Field

Telegrams include an error-checking field, operating based
on a cyclic redundancy check (CRC) method. The CRC field
checks the contents of the entire telegram. It is applied
regardless of any parity check method used for the
individual characters of the telegram. The transmitting
device calculates the CRC value and appends the CRC as
the last field in the telegram. The receiving device
recalculates a CRC during receipt of the telegram and
compares the calculated value to the actual value received
in the CRC field. Two unequal values result in bus timeout.
The error-checking field contains a 16-bit binary value
implemented as 2 8-bit bytes. After the implementation,
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 telegram.

7.8.8 Coil Register Addressing

In Modbus, all data is organized in coils and holding
registers. Coils hold a single bit, whereas holding registers
hold a 2 byte word (that is 16 bits). All data addresses in
Modbus telegrams are referenced to 0. The first occurrence
of a data item is addressed as item number 0. For example:
The coil known as coil 1 in a programmable controller is
addressed as coil 0000 in the data address field of a
MG18N202