700-SDI-RADAR-300W-Man Rev 2 30 May 2022
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Part# 21370
CSUM: Checksum, calculated by adding in modulo 256 the two byte values of the ID.
If the device ID is 2, then ID was sent as <0x30><0x32>.
Checksum is then 0x30+0x32 = <0x62>.
After receiving the request, if the device ID matches, the flow meter will respond with the current
averaged flow velocity reading:
<0xA5> ID SPEED CSUM
0xA5: The first byte sent in the response is byte with HEX value of 0xA5.
ID: Exactly two bytes long. This is the unit ID written as two ASCII characters.
For example, if the unit ID is 2, then ID will be sent as “02”. In HEX representation
it is the following two bytes: <0x30><0x32>.
SPEED: The speed readout in currently selected units, formatted as real (float) number
With exactly three digits after the decimal dot separator.
For example, if the current averaged speed is 5.7143, it will be reported as
5.714, or in HEX values: <0x35><0x2E><0x37><0x31><0x34><0x33>.
CSUM: Checksum, calculated by adding in modulo 256 the two byte values of the ID and
All byte values from the SPEED.
4.2.4
MODBUS PROTOCOL (RS-485)
When configured in Modbus operation mode, the unit responds to Modbus requests over the 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 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 WL sensor) responds. The request and response format, with
example, is given in tables 4-1 through 4-4. 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 4-1: Master Request Format
Name
Addr
Fun
Data start Addr
Data#of regs
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
