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11. Modbus telegrams and function codes
11.1 Modbus telegram overview
The maximum size of a Modbus RTU telegram is 256 bytes.
Telegrams must be separated by a silent interval of at least
3.5 character times.
The standard Modbus RTU telegram format is shown in the table
below:
A telegram starts with the slave address occupying one byte. The
function code comes next. See section
for supported function codes. Then comes a variable-
size data field. For each telegram, a CRC is calculated and
appended to the telegram (two bytes total). All bytes in the
telegram, except for the CRC itself, are included in the check.
Slave address
Function code
Data
CRC
1 byte
1 byte
0 to 252 bytes
2 bytes
Note
The CRC bytes are not shown in the examples in
the following sections.