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15. Modbus telegram examples
15.1 Diagnostics: return query data
This function is useful to ensure that the communication path and
slave configuration are correct. It will echo the request in the
response.
In the example, slave address 0x01 is used.
Request from master to slave
Example of response from slave to master
If there is no response from the slave, see section 16.1.2 CIM
200 Modbus communication faults.
15.2 Reading the CIM configuration register block
This section shows how to read the CIM configuration register
block (a total of four registers).
In the example, slave address 0x01 is used.
Request from master to slave
Example of response from slave to master
If there is no response from the slave, see section 16.1.2 CIM
200 Modbus communication faults.
15.3 Reading the pit water level
This section shows how to read and interpret the water level of
the pit.
In the example, slave address 0x01 is used.
Request from master to slave
Example of response from slave to master
A pit water level value of 220 [0.01 m] equals a water level of 2.20
metres.
If there is no response from the slave, see section 16.1.2 CIM
200 Modbus communication faults.
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.
Field Value Description
Slave address 0x01 -
Function code 0x08 Diagnostics
Subcode 0x00 Echo request
Data 0xAB Test data
Data 0xCD Test data
Field Value Description
Slave address 0x01 -
Function code 0x08 Diagnostics
Subcode 0x00 Echo request
Data 0xAB Test data
Data 0xCD Test data
Field Value Description
Slave address 0x01 -
Function code 0x04 Read input registers
Start address HI 0x00
Start address
= 0x0001
Start address LO 0x00
Quantity HI 0x00
Number of registers
= 0x0004
Quantity LO 0x04
Field Value Description
Slave address 0x01 -
Function code 0x04 Read input registers
Byte count 0x08 8 bytes follow
00001 HI 0x0A
SlaveMinimumReplyDelay
= 0x000A (10 ms)
00001 LO 0x00
00002 HI 0x00
RegisterOffset
= 0x0000 (no offset)
00002 LO 0x00
00003 HI 0x00
Reserved value
= 0x0000
00003 LO 0x00
00004 HI 0x00
ModbusBitRate
= 0x0004 (19200, software-defined)
00004 LO 0x04
Field Value Description
Slave address 0x01 -
Function code 0x04 Read input registers
Start address HI 0x01
Start address (00301)
= 0x012D
Start address LO 0x2C
Quantity HI 0x00
Number of registers
= 0x0001
Quantity LO 0x01
Field Value Description
Slave address 0x01 -
Function code 0x04 Read input registers
Byte count 0x02 2 bytes follow
00301 HI 0x00
Pit water level
= 0x00DC (220)
00301 LO 0xDC
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