Burkert Type 8717 - Manual (Page 67)

67
Starting up the Modbus
the request:
The function code in the request informs the addressed slave which action is to be executed. The data bytes
include all additional information that the slave requires to execute the action.
E.g. if the function code 03 requests the slave to read out the holding register and to send back its contents. The
data field must include the following information: Start register and the number of registers to be read. In this case
one register corresponds to one WORd (2 bytes). The slave can use the checksum to determine the validity of
the telegram contents.
the reply:
The structure of the reply corresponds to the request telegram one. If an error occurs, an error code is sent
instead of the function code. In this case the data includes a code which describes the error. The master can use
the checksum to determine the validity of the telegram contents.
example of modbus communication (Read Input Register commands)
The request specifies the initial register and the number of input registers to be read.
In the following example the value of the totalizer is requested from the device with address 1.
Request
field name value
Slave address 0x01
Function 0x04 (Read Input Register)
Initial address high 0x00
Initial address low 0x0A
Number of high registers 0x00
Number of low registers 0x02
Error check CRC (high byte)
Error check CRC (low byte)
The register data in the reply is compressed as two bytes per register.
The reply is transferred as soon as the data has been completely assembled.
Here is an example of the reply to the previous request:
field name value
Slave address 0x01
Function 0x04
Byte count 0x04
data1 high byte 0x00
data1 low byte 0x00
data2 high byte 0x09
data2 low byte 0x04
Error check CRC (high byte)
Error check CRC (low byte)
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MFC Family