TECHTOP TD20 Series Product Manual page 100

TD20 Series VFD
10-bit character frame (BIT1–BIT7 are the digital 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.
The Modbus minimum idle time between frames should be no less than 3.5 bytes. The network device is
detecting, even during the interval time, the network bus. When the first field (the address field) is
received, the corresponding device decodes next transmitting character. When the interval time is at
least 3.5 byte, the message ends.
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
7.2.2.2 RTU communication frame error checkout
Various factors (such as electromagnetic interference) may cause error in the data transmission. For
example, if the sending message is logic "1", A-B potential difference on RS485 should be 6V, but in
reality, it may be -6V because of electromagnetic interference, and then the other devices take the sent
message as logic "0". 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
BIT2 BIT3 BIT4
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 x N bytes are the main content of the communication as well
as the core of data exchanging
Detection value: CRC (16 BIT)
T1-T2-T3-T4 (transmission time of 3.5 bytes)
BIT5 BIT6
-98-
Communication Protocol
Check
BIT7 End bit
bit