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13. Modbus RTU telegram examples
13.1 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.
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.
13.2 Read holding registers (0x03)
This function is used for reading holding registers from the slave.
The request telegram specifies the starting address (the address
of the first register to be read) and the number of holding
registers to read. In the telegram, register addresses start from
zero, meaning that registers numbered 0-16 are addressed as
0-15.
The register data in the response message are packed two bytes
per register. For each register, the first byte contains the high-
order bits while the second byte contains the low-order bits.
Example of request from master to slave
In the request, the slave with address 1 is asked to deliver three
contiguous registers starting from address 0x006b = 107
(meaning register 108).
Example of response from slave to master
In the response, the byte count is six since there are three
registers of two bytes. All three registers hold the value of
0x0001.
13.3 Read input registers (0x04)
This function is used for reading input registers from the slave.
Input registers are read-only registers by definition. The request
telegram specifies the starting address (the address of the first
register to be read) and the number of holding registers to read.
In the telegram, register addresses start from zero, meaning that
registers numbered 0-16 are addressed as 0-15.
The register data in the response message are packed two bytes
per register. For each register, the first byte contains the high-
order bits while the second byte contains the low-order bits.
Example of request from master to slave
In the request, the slave with address 1 is asked to deliver three
contiguous registers starting from address 0x1010 = 4112
(meaning register 4113).
Example of response from slave to master
In the response, the byte count is six since there are three
registers of two bytes. All three registers hold the value of
0x2222.
CRC fields are not shown in the following
examples.
The Modbus data model states that registers
numbered X are addressed in telegrams as X - 1,
e.g. register 00104 (setpoint) is addressed as
00103 in a Modbus telegram.
Slave
address
Function
code
Data CRC
1 byte 1 byte 0 to 252 bytes 2 bytes
The CRC bytes are not shown in the examples in
the following sections.
Field Value
Address 0x01
Function code 0x03
Start address HI 0x00
Start address LO 0x6B
Quantity HI 0x00
Quantity LO 0x03
Field Value
Address 0x01
Function code 0x03
Byte count 0x06
Register 108 HI 0x00
Register 108 LO 0x01
Register 109 HI 0x00
Register 109 LO 0x01
Register 110 HI 0x00
Register 110 LO 0x01
Field Value
Address 0x01
Function code 0x04
Start address HI 0x10
Start address LO 0x10
Quantity HI 0x00
Quantity LO 0x03
Field Value
Address 0x01
Function code 0x04
Byte count 0x06
Register 4113 HI 0x22
Register 4113 LO 0x22
Register 4114 HI 0x22
Register 4114 LO 0x22
Register 4115 HI 0x22
Register 4115 LO 0x22
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