Geolux RSS-2-300W, User Manual, Page: 27 / 41

RSS-2-300W User Manual
24 RSS-2-300W Non-Contact Surface Velocity Radar
7.4. Modbus Protocol (RS-485)
When configured in Modbus operation mode, the unit responds to Modbus requests over RS-485
dana line. The baud rate and device ID configured through the PC application, and 1 stop bit, even
parity, 8 data bits configuration is used.
Modbus registers that are accessed by Modbus protocol are 16-bit (2-byte) registers. Any number
of registers can be read or written over Modbus.
Modbus is a request-response protocol where a master (such as datalogger) sends out requests,
and slave devices (such as RSS-2-300 W sensor) responds. The request and response format, with
examples is given in tables 3-6.
In each request, the master can either ask the slave to retrieve value of one or more registers, or
the master can set the value of one or more registers. Each register holds one 16-bit value.
Table 3. Master Request Format
Name Address Fun Data Start
Address
Register Count CRC16
Length 1 byte 1 byte 2 bytes (H,L) 2 bytes (H,L) 2 bytes (L,H)
Example 0X01 0X03 0X00 0X00 0X00 0X01 0X84 0X0A
Table 4. Request Example
Name Content Detail
Address 0X01 Slave address (Sensor id)
Function 0X03 Read slave info
Data start address 0X00 The address of the first register to read (HIGH)
0X00 The address of the first register to read (LOW) – Sensor ID reg
Number of regs 0X00 High
0X01 Low (read only 1 register)
CRC16 0X84 CRC Low
0X0A CRC High
Table 5. Slave (sensor) Response Format
Name Address Fun Byte Count Data CRC16
Length 1 byte 1 byte 1 byte 2 bytes(H,L) 2 bytes(L,H)
Example 0X01 0X03 0X02 0X00 0X01 0X79 0X84