Goodrive350A series high-performance multifunction VFD
Communication
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Use shielded cables, if possible, in multi-VFD connection. The baud rates, data bit check settings,
and other basic parameters of all the devices on the RS485 line must be set consistently, and
addresses cannot be repeated.
9.3.2 RTU
9.3.2.1 RTU communication frame structure
When a controller is set to use the RTU communication mode on a Modbus network, every byte (8
bits) in the message includes 2 hexadecimal characters (each includes 4 bits). Compared with the
ASCII mode, the RTU mode can help to send more data at the same baud rate.
Code system
•
1 start bit
•
7 or 8 data bits; the minimum valid bit is sent first. Each frame domain of 8 bits includes 2
hexadecimal characters (0
–9, A–F).
•
1 odd/even check bit; this bit is not provided if no check is needed.
•
1 stop bit (with check performed), or 2 bits (without check)
Error detection domain
•
Cyclic redundancy check (CRC)
The following table describes the data format.
11-bit character frame (Bits 1 to 8 are data bits)
Start bit
BIT1
BIT2
BIT3
BIT4
BIT5
BIT6
BIT7
BIT8
Check
bit
Stop bit
10-bit character frame (Bits 1 to 7 are data bits)
Start bit
BIT1
BIT2
BIT3
BIT4
BIT5
BIT6
BIT7
Check
bit
Stop bit
In a character frame, only the data bits carry information. The start bit, check bit, and stop bit are used
to facilitate the transmission of the data bits to the destination device. In practical applications, you
must set the data bits, parity check bits, and stop bits consistently.
In RTU mode, a new frame always must be preceded by a time gap with a minimum length of 3.5
bytes. On a network where the transmission rate is calculated based on the baud rate, the
transmission time of 3.5 bytes can be easily obtained. After the idle time ends, the data domains are
sent in the following sequence: slave address, operation command code, data, and CRC check
character. Each byte sent in each domain includes hexadecimal characters (0
–9, A–F). The network
devices always monitor the communication bus. After receiving the first domain (address information),
each network device identifies the byte. After the last byte is sent, a similar transmission interval (with
a minimum length of 3.5 bytes) is used to indicate that the frame transmission ends. Then, the
transmission of a new frame starts.
