Dräger Polytron 8000 Series Technical Manual page 11

Communication in the Modbus network occurs through so-called polling. The
Master sends a telegram (Request) to the Slave. It then receives the Response.
Data transmission in Modbus RTU is in binary format. A data telegram consists of
several blocks (Frames):
– One address block (1 byte = 1 character)
– One function block (1 byte = 1 character)
– One data block (variable)
– One checksum block (CRC = Cyclic Redundancy Check)
Between 2 telegrams there has to be a break with a length of at least
3.5 characters. The corresponding wait time depends on the transmission rate. If a
transmission break of more than 1.5 characters occurs during telegram
transmission, the transmission process is considered incomplete and should be
aborted.
General form of a Modbus telegram
Device address
8 bits
At least 3.5 char
Address
The address specifies which device is contacted (Master - Slave) or which device
responds. The address is a number between 1 and 126. The address 0 is used for
messages to all devices (Broadcast), provided that the selected function supports
this.
Field
Address
CRC16
The configuration of the address depends on the respective device and the system
architecture.
|
Technical Manual
Digital Communication - Modbus RTU
Function code
8 bits
Start Address
≥ 3.5 char
8 bits
Frame 1
At least 3.5 char
Length Description
1 byte Identifies the target
device
2 byte Checksum (CRC16)
Basic principles of the Modbus technology
Data
n* 8 bits
Modbus telegram
Function
Data CRC
code
8 bits N x 8 bits 16 bits
Frame 1
3.5 char
4.5 char
Master
Set by the Mas-
ter
Calculated by
the Master
CRC check
16 bits
End
≥ 3.5 char
Frame 1
Slave
Copied from the
request into the
response
Calculated by the
Slave
11