Astraada DRV-24 frequency inverters Communication protocol
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Figure 3 star connection
It is recommended to use shield cables in multiple connection. The basic parameter of the devices,
such as baud rate and digital check bit in RS485 should be the same and there should be no
repeated address.
7.2.2 RTU mode
7.2.2.1 RTU communication frame format
If the controller is set to communicate by RTU mode in Modbus network every 8bit byte in the
message includes two 4Bit hex characters. Compared with ACSII mode, this mode can send more
data at the same baud rate.
Code system
· 1 start bit
· 7 or 8 digital bit, the minimum valid bit can be sent firstly. Every 8 bit frame includes two hex
characters (0...9, A...F)
· 1 even/odd check bit . If there is no checkout, the even/odd check bit is inexistent.
· 1 end bit (with checkout), 2 Bit(no checkout)
Error detection field
· CRC
The data format is illustrated as below:
11-bit character frame (BIT1~BIT8 are the digital bits)
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:
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