Azbil F4Q, Руководство пользователя

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5-2
Chapter 5. Modbus Communication (RTU Mode)
5 - 2 Message Structure
„
Overview of Modbus-RTU Frame
All messages use binary data.
• Modbus-RTU communication frame
Frame structure Number of bytes Description
(Start) 0 Silent interval
Address 1 Device address
Function 1 For details, see on the particular function
code
Data n* For details, see on the particular function
code
CRC check 2 Check code
(End) 0 Silent interval
* From 1 to 252
Handling Precautions
•
If the device address is set to 0, broadcast communication will be performed.
The address is not used for normal communication.
„ Silent interval
In the Modbus-RTU protocol, the period equivalent to a minimum of 3.5 characters
(depending on the transmission speed) without communication is prescribed to
mark the start and end of command and response messages.
For this device, the length of the silent interval is defined as follows.
Transmission speed Period equivalent to 3.5
characters
Silent interval for this device
38400 bps 1.003 ms 2 ms or more
19200 bps 2.005 ms 3 ms or more
9600 bps 4.010 ms 5 ms or more
4800 bps 8.021 ms 9 ms or more
One character is 11 bits (start bit + data bit + parity bit + stop bit)
If the silent interval is shorter than the above, the end of a frame may not be
recognized. When sending multiple messages, allow a enough interval between
frames.
„ Function code
Of the Modbus protocol public function codes, the following are supported.
Code (Hex) Description
03 0x03 Read Holding Registers (read multiple data records)
06 0x06 Write Single Register (write one data record)
16 0x10 Write Multiple Registers (write multiple data records)