Pos. Description
1 Clients
2 SCADA system
3 PC Tool, etc.
4 LAN
5 GRE router
6 VPN tunnel
7 Internet
8 Cellular operator
9 Cellular network
10 Base station APN
11 "Static" IP address
12 CIU 260 with CIM 260 module and SIM card
13 Cellular network
14 Setup, status and control commands via SMS
15 Mobile phone
Related information
5.5 Status LEDs
13. Modbus RTU telegram examples
The Modbus data model states that registers numbered X
are addressed in telegrams as X - 1, for example register
00104 (setpoint) is addressed as 00103 in a Modbus
telegram.
Note that CRC fields are not shown in the following 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.
Slave address Function code Data CRC
1 byte 1 byte 0 to 252 bytes 2 bytes
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.
Note that the CRC bytes are not shown in the examples in the
following sections.
13.2 Read holding registers, 0x03
This function is used for reading holding registers from the slave.
The request telegram specifies the starting address, that is 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.
Example of request from master to slave
Field
Value
Address 0x01
Function code 0x03
Start address HI 0x00
Start address LO 0x6B
Quantity HI 0x00
Quantity LO 0x03
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
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
In the response, the byte count is six since there are three registers
of two bytes. All three registers hold the value of 0x0001.
37
English (GB)
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