FISCHER DE43, Руководство по эксплуатации

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4.3 Modbus functional description
The following describes the Modbus functions that support the DE43 Transmit-
ter. The descriptions are initially kept general as explained in the
Modbus Pro-
tocol Specification and are then supplemented with the unique features of the
DE43.
4.3.1 Modbus Transaction
A Modbus transaction comprises two parts. A request from the Master and a re-
sponse from the Slave.
Master Slave
Init Request
Function Code Data Request
Instruction executed
&
Modbus response
Receive Response
Function Code Data Response
Fig. 19: Error-free request/response cycle
If an error occurs during a Modbus transaction, the Function Code is replaced
with a special Function Code with an error indicator in the Modbus Response
message and a more detailed description of the error in the data field is sent.
Master Slave
Init Request
Function Code Data Request
Error detected
&
Modbus response
Receive Response
Error Code Function Code (error)
Fig. 20: Faulty request/response cycle
4.3.2 Modbus telegram
The maximum size of a Modbus telegram is 256Byte. The telegram comprises a
1 byte slave address, a 1 byte function code, the data and a 2 byte CRC-check-
sum.
Frame 1
3.5 char 3.5 char
4.5 char
Frame 2
3.5 char
Frame 3
t
0
Starting
≥3.5 char
Address
8 bit
Function
8 bit
Data
N x 8 bit
CRC Check
16 bits
End
≥3.5 char
Modbus message
Fig. 21: Modbus Message Frame