INVT Goodrive3000 Series Operation Manual page 254

Goodrive3000 series medium voltage VFD
11-bit character frame (BIT1–BIT8 are the data bits)
Start bit BIT1 BIT2
10-bit character frame (BIT1–BIT7 are the data bits)
Start bit BIT1
In one character frame, the digital bit takes effect. The start bit, check bit and end bit is used to send the
digital bit right to the other device. The digital bit, even/odd checkout and end bit should be set as the
same in real application.
In the RTU mode, the minimum idle time between new frames should be no less than 3.5 bytes. In the
network whose transmission speed is calculated by baud rate, transmission time of 3.5 bytes can be
controlled easily. The data fields are as follows: slave address, operation code, data and CRC checkout,
the byte of each field is hex (0...9, A...F). The network device is always monitoring the action of
communication bus. When the first field (the address message) is received, each device will confirm the
byte. After the final byte is transmitted, there will be another interval time similar to 3.5 bytes to indicate
the end of the frame. Later, a new frame will start.
Start with a time gap
(with a min. length of 3.5
The whole message frame in RTU mode is a continuous transmitting flow. If there is an interval time
(more than 1.5 bytes) before the completion of the frame, the receiving device will renew the
uncompleted message and suppose the next byte as the address field of the new message. As such, if
the new message follows the previous one within the interval time of 3.5 bytes, the receiving device will
deal with it as the same with the previous message. If these two phenomena all happen during the
transmission, the CRC will generate a fault message to respond to the sending devices.
The standard structure of RTU frame:
START
ADDR
CMD
DATA (N-1)
...
DATA (0)
CRC CHK low bit
CRC CHK high bit
END
8.3.2.2 RTU communication frame error checkout
Various factors may cause error in the data transmission. If there is no error checkout, the receiving
devices will not find the message is wrong and they may give incorrect response which cause serious
BIT3 BIT4
BIT2 BIT3 BIT4
RTU data frame format
Modbus packet
Slave Function
address code
bytes)
T1-T2-T3-T4 (transmission time of 3.5 bytes)
Communication address: 0–247(decimal system)(0 is the broadcast
address)
03H:read slave parameters
06H:write slave parameters
The data of 2*N bytes are the main content of the communication as
well as the core of data exchanging
Detection value:CRC (16BIT)
T1-T2-T3-T4 (transmission time of 3.5 bytes)
BIT5 BIT6 BIT7
BIT5 BIT6
End with a time gap
Data Check (with a min. length of
-249-
MODBUS protocol
BIT8 Check bit End bit
BIT7 Check bit End bit
3.5 bytes)