Icon Process Controls ProCon DO3000-C Series Quick Start Manual page 19

ProCon — DO3000-C Series
®
Dissolved Oxygen Controller
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)
Check Domain Structure: Cyclic Redundancy Check (CRC16)
Structure description:
Slave Instrument
Address
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.
≥ 3.5 bytes
Starting
≥ 3.5 bytes
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.
25-0671 © Icon Process Controls Ltd.
Start bit 1 2 3
Fig.3 : RTU Pattern Bit Sequence
Function Code
1 byte
Fig.4 : RTU Information Structure
Frame 1
≥ 3.5 bytes
Address Function code
8
Fig.5 : RTU Message Frame
4 5 6 7 8 Stop bit
Data
0...252 byte
Frame 2
3.5 bytes
4.5 bytes
Data
8 Nx8
Stop bit
CRC
2 byte
CRC Low byte | CRC High byte
Frame 3
CRC End
16 bit ≥ 3.5 bytes
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