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8.8.2 Modbus RTU Telegram Structure
The transmitting device places a Modbus RTU telegram into a frame with a known beginning and ending point. This allows receiv-
ing devices to begin at the start of the telegram, read the address portion, determine which device is addressed (or all devices, if the
telegram is broadcast), and to recognize when the telegram is completed. Partial telegrams are detected and errors set as a result.
Characters for transmission must be in hexadecimal 00–FF format in each field. The drive continuously monitors the network bus,
also during silent intervals. When the 1
st
field (the address field) is received, each drive or device decodes it to determine which
device is being addressed. Modbus RTU telegrams addressed to 0 are broadcast telegrams. No response is allowed for broadcast
telegrams. A typical telegram frame is shown in the following table.
Table 62: Typical Modbus RTU Telegram Structure
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
8.8.3 Start/Stop Field
Telegrams 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 1
st
field to be transmitted is the device address. Fol-
lowing the last transmitted character, a similar period of at least 3.5 character intervals marks the end of the telegram. A new tele-
gram 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 com-
pletion 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 con-
siders it a continuation of the previous telegram. This behavior causes a timeout (no response from the follower), since the value in
the final CRC field is not valid for the combined telegrams.
8.8.4 Address Field
The address field of a telegram frame contains 8 bits. Valid follower device addresses are in the range of 0–247 decimal. The individ-
ual follower devices are assigned addresses in the range of 1–247. 0 is reserved for broadcast mode, which all followers recognize. A
master addresses a follower by placing the follower address in the address field of the telegram. When the follower sends its re-
sponse, it places its own address in this address field to let the master know which follower is responding.
8.8.5 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 tele-
grams between master and follower. When a telegram 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 (errorfree) 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 telegram. This code tells the master what kind of error occurred, or the reason for
the exception. Also refer to
8.8.10.2 Function Codes Supported by Modbus RTU
8.8.10.3 Modbus Exception Codes
.
8.8.6 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 follower device contains additional information which the follow-
er 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.
8.8.7 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 tele-
gram. 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. 2
unequal values result in bus timeout. The error-checking field contains a 16-bit binary value implemented as 2 8-bit bytes. After the
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RS485 Installation and Set-up
VLT® Flow Drive FC 111
Design Guide