Geolux LX-80-15, User Manual

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LX-80-15 10 Hz User Manual
33 LX-80-15 10 Hz Oceanographic Radar Sensor
8.3. Modbus Protocol (RS-485)
The unit responds to Modbus requests over RS-485 data line. The baud rate is 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 to over Modbus.
Modbus is a request-response protocol where a master (such as datalogger) sends out requests, and
slave devices (such as LX-80 sensor) respond. The request and response format, with examples is given
in Tables 4-7.
In each request, the master can either ask the slave to retrieve the 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 4. 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 5. Request Example
Name Content Detail
Address 0X01 Slave address (Sensor id)
Function 0X03 Read holding register
Data start address 0X00 The address of the first register to read minus one (HIGH)
0X00 The address of the first register to read minus one (LOW)
Number of regs 0X00 High
0X01 Low (read only 1 register)
CRC16 0X84 CRC Low
0X0A CRC High
Table 6. 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 0X0A
Please note that Modbus register addresses start with 1 i.e. the lowest possible address for the Modbus
register is 1. However, in the Modbus request packet the data start address is written as the address of
the first register to read minus one. For example, to retrieve the Modbus register number 5, in Modbus
data packet the address of that register should be written as 4.