Icon Process Controls ProCon DO3000-C Series Quick Start Manual page 20

ProCon — DO3000-C Series
®
Dissolved Oxygen Controller
The RTU mode contains an error-detection domain based on a cyclic redundancy check (CRC) algorithm that performs
on all message contents. The CRC domain checks the contents of the entire message and performs this check
regardless of whether the message has a random parity check. The CRC domain contains a 16-bit value consisting of
two 8-bit bytes. CRC16 check is adopted. Low bytes precede, high bytes precede.
Implementation of MODBUS RTU in Instrument
According to the official MODBUS definition, the command starts with a 3.5 character interval triggering command,
and the end of the command is also represented by a 3.5 character interval. The device address and MODBUS function
code have 8 bits. The data string contains n*8 bits, and the data string contains the starting address of the register and
the number of read/write registers. CRC check is 16 bits.
Value Start
No Signal bytes
during 3.5 Characters
Byte 3.5
Instrument MODBUS RTU Function Code
The instrument only uses two MODBUS function codes:
0x03: Read-and-hold register
0x10: Write multiple registers
MODBUS Function Code 0x03: Read-and-hold Register
This function code is used to read the continuous block content of the holding register of the remote device. Request
the PDU to specify the start register address and the number of registers. Address registers from zero. Therefore, the
addressing register 1-16 is 0-15. The register data in the response information is packaged in two bytes per register. For
each register, the first byte contains high bits and the second byte contains low bits.
Request:
Function Code
Start Address
Read Register Number
25-0671 © Icon Process Controls Ltd.
Frame 1 normal
< 1.5 bytes
Fig.6 : MODBUS RTU CRC Check
Device Address Function
Function
Codes
1-247
Confirming
1
to MODBUS
Specification
Fig.7 : MODBUS definition of Data Transmission
1 byte
2 byte
2 byte
Fig.8 : Read and hold register request frame
Frame 2 fault
> 1.5 bytes
Data
Data
Confirming
to MODBUS
Specification
1 N
Summary
End
Check
No Signal bytes
CRCL CRCL during 3.5
characters
1 1 3.5
0x03
0x0000....0xfffff
1...125
20