Rtu Mode - IMO HD2 IP Series Instruction Manual

HD2 IP55 High-ingress Protection Series Inverter
Use shielded cable, if possible, in multi-device 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 mode

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 transmit more data with the same baud rate.
Code system
1 start bit

7 or 8 data bits; the minimum valid bit is transmitted 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 end bit (with check performed), 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
10-bit character frame (Bits 1 to 7 are data bits)
Start bit BIT1
In a character frame, only the data bits carry information. The start bit, check bit, and end 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 end bits consistently.
In RTU mode, the transmission of a new frame always starts from an idle time (the transmission
time 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 transmitted in the following sequence: slave address, operation command code,
data, and CRC check character. Each byte transmitted in each domain includes 2 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 transmitted, a similar transmission interval (the transmission time of 3.5 bytes)
is used to indicate that the transmission of the frame ends. Then, the transmission of a new
frame starts.
Start with an idle time (at
least the transmission
time of 3.5 bytes)
BIT2 BIT3 BIT4
BIT2 BIT3 BIT4
RTU data frame format
Modbus packet
Slave Function
address code
BIT5 BIT6 BIT7
BIT5 BIT6
Data Check
-264
BIT8 Check bit End bit
BIT7 Check bit End bit
End with an idle time (at
least the transmission
time of 3.5 bytes)