Modbus Telegram - Endress+Hauser Proline Promass 84 MODBUS RS485 Operating Instructions Manual

Operation
38
5.5.2

MODBUS telegram

General
The master-slave process is used for data exchange. Only the master can initiate data transmission.
Following the prompt, the slave sends the master the necessary data as a response telegram or
executes the command requested by the master.
Telegram structure
The data is transferred between the master and slave by means of a telegram.
A request telegram from the master contains the following telegram fields:
Telegram structure:
Slave address
• Slave address
The slave address can be in an address range from 1 to 247.
The master talks to all the slaves simultaneously by means of the slave address 0 (broadcast
message).
• Function code
The function code determines which read, write and test operations should be executed by means
of the MODBUS protocol.
Function codes supported by the measuring device → ä 39
• Data
Depending on the function code, the following values are transmitted in this data field:
– Register start address (from which the data are transmitted)
– Number of registers
– Write/read data
– Data length
– etc.
• Check sum (CRC or LRC check)
The telegram check sum forms the end of the telegram.
The master can send another telegram to the slave as soon as it has received an answer to the
previous telegram or once the time-out period set at the master has expired. This time-out period
can be specified or modified by the user and depends on the slave response time.
If an error occurs during data transfer or if the slave cannot execute the command from the master,
the slave returns an error telegram (exception response) to the master.
The slave response telegram consists of telegram fields which contain the requested data or which
confirm that the action requested by the master has been executed. It also contains a check sum.
Function code
Proline Promass 84 MODBUS RS485
Data Check sum
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