Kübler Group
5 Commissioning and Operation
HB Modbus Interface - R67048.0002 - 02
EN - 13
40271
270
Serial Number
MSB
-
40272
271
Serial Number
LSB
-
40273
272
Serial Update
MSB
0x00
40274
273
Sensor Diag
MSB
0x00
40275
274
Lower Limit
MSB
0x00
40276
275
Upper Limit
MSB
0x1C
40277
276
Compare Activ.
MSB
0x00
40278
277
MUR
MSB
0x0001
40279
278
MUR
LSB
0x0000
40280
279
TMR
MSB
0x1000
40281
280
TMR
LSB
0x0000
40282
281
Scaling Function
MSB
0x00
40283
282
Delay Prescaler
MSB
0x01
5.3 Protocol Features
5.3.1 Structure of the Modbus RTU frames
To carry out changes in the device, the respective Modbus registers must be addressed through
the telegram. The basic structure of a Modbus telegram is shown below:
Start
Address
Function
Data
CRC
Stop
3.5 bytes
1 byte
1 byte
N x 8 bits
2 bytes
3.5 bytes
The data range has a different structure depending on whether the telegram is a query or a re-
sponse and on the used function code.
In RTU mode, the messages start with silent interval of at least 3.5 characters.
According to the baud rate set in the network, this interval is easiest implemented as a multiple
of the duration of a character.
The first field subsequently transmitted is the device address in the range of 01…0xF7 (247)
(248-255 are reserved for Modbus). Characters permissible for all fields: hexadecimal 0–9, A–F.
The networked devices constantly monitor the network bus - also during the 'silent' intervals.
When the first field (address field) is received, the sensor decodes it to determine whether the
message is directed to it.
After the last transmitted character, an identical interval of at least 3.5 characters indicates the
end of the message. A new message can start after this interval.
The complete message frame must be transmitted as a continuous data stream. In the event of
a silent interval of more than 1.5 characters before the end of the frame, the receiver device
erases the message and assumes that the following byte is the address of a new message.
Likewise, if a new message starts before the end of the silent interval of 3.5 characters, the re-
ceiver device considers this new message as the continuation of the previous message. This
will trigger a fault, as the value in the final CRC field will not be valid for the combined mes-
sages.