INVT Goodrive3000 GD3000-01-055G-12 Operation Manual page 204

Goodrive3000 series medium voltage inverters
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.
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
result. So the checkout is essential to the message.
The theme of checkout is that: the sender calculate the sending data according to a fixed formula, and
then send the result with the message. When the receiver gets this message, they will calculate anther
result according to the same method and compare it with the sending one. If two results are the same,
the message is correct. If not, the message is incorrect.
The error checkout of the frame can be divided into two parts: the bit checkout of the byte and the whole
data checkout of the frame (CRC check).
Bit checkout of the byte
The user can select different bit checkouts or non-checkout, which impacts the check bit setting of each
byte.
The definition of even checkout: add an even check bit before the data transmission to illustrate the
number of "1" in the data transmission is odd number or even number. When it is even, the check byte is
"0", otherwise, the check byte is"1". This method is used to stabilize the parity of the data.
The definition of odd checkout: add an odd check bit before the data transmission to illustrate the number
of "1" in the data transmission is odd number or even number. When it is odd, the check byte is "0",
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)
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MODBUS protocol