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Application Guide | iC2-Micro Frequency Converters
Table 34: Typical Modbus RTU Telegram Structure
Start
Address
T1-T2-T3- T4
8 bits
Start/Stop Field
6.1.7.4.3
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 1st 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 follower), since the value in the final
CRC field is not valid for the combined telegrams.
Address Field
6.1.7.4.4
The address field of a telegram 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 recognize. A master
addresses a follower by placing the follower address in the address field of the telegram. When the follower sends its response, it places
its own address in this address field to let the master know which follower is responding.
Function Field
6.1.7.4.5
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 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 (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 telegram. This code tells the master what kind of error occurred, or the reason for the exception. Also refer
to
6.2.2 Function Codes Supported by Modbus RTU
Data Field
6.1.7.4.6
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 follower must
use to perform accordingly.
The information can include items such as:
Coil or register addresses.
l
The quantity of items to be handled.
l
The count of actual data bytes in the field.
l
86
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Function
Data
8 bits
N x 8 bits
and
6.2.3 Modbus Exception
CRC check
16 bits
Codes.
AB413939445838en-000301 / 130R1254
End
T1-T2-T3- T4
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