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C500 | C520 Series Operating Manual
MODBUS RTU Transmission Mode
When the instrument uses RTU (Remote Terminal Unit) mode for MODBUS serial communication, each 8-bit byte of information contains
two 4-bit hexadecimal characters. The main advantages of this mode are greater character density and better data throughput than the
ASCII mode with the same baud rate. Each message must be transmitted as a continuous string.
The format of each byte in RTU mode (11 bits):
Coding system: 8-bit binary
Each 8-bit byte in a message contains two 4-bit hexadecimal characters (0-9, A-F)
Bits in each byte: 1 starting bit
8 data bits, the first minimum valid bits without parity check bits
2 stop bits
Baud rate: 9600 BPS
How characters are transmitted serially:
Each character or byte is sent in this order (from left to right) the least significant bit (LSB)... Maximum
Significant Bit (MSB)
Start bit
1
Figure 3: RTU pattern bit sequence
Check Domain Structure: Cyclic Redundancy Check (CRC16)
Structure description:
Slave Instrument
Function Code
Address
1 byte
Figure 4: RTU information structure
The maximum frame size of MODBUS is 256 bytes MODBUS RTU Information Frame In RTU mode, message frames are distinguished by
idle intervals of at least 3.5 character times, which are called t3.5 in subsequent sections.
Start
Address
≥3.5 bytes
8
Figure 5: RTU Message Frame
The entire message frame must be sent in a continuous character stream. When the pause time interval between two characters exceeds
1.5 characters, the information frame is considered incomplete and the receiver does not receive the information frame.
Figure 6: Frame data transmission
2
3
4
Data
0...252 byte
Function code
8
5
6
7
CRC
2 byte
CRC Low byte
CRC Low byte
Date
CRC
Nx8
16
◀ ◀
8
Stop bit
Stop bit
Stop
≥3.5 bytes
24
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