Function code: 03

Function code 03 of Modbus communication protocol is used to read numerical value (1 value) of the sensor or display. Host command is organized in a sequence from machine address, function code, start address, number of bytes to CRC code. For slave computer, the response command is organized in the sequence from machine address, function code, data area to CRC code. Data is the data area is binary presented by two bytes with the highest byte in the front. CRC code includes two bytes with the lowest byte in the front.

Information frame format: (address of the slave computer is 01, which are binary data)

Host sending: station number (1B) function code (1B) start address (2B) reading points CRC (2B)
T1~T4 01 03 00 0X 00 0X XX XX T1~T4

In which: for any frame beginning and ending with T1~T4, 3~5 quiescent stages should be remained;
Station number (address): one byte “01”
Function code: one byte “03”,
Start address: two bytes, one of 0000~0003 is applicable;
0000, returning radar distance value at present (cm)
0001, returning radar distance value at present (m)
0002, returning radar level value at present (cm)
0003, returning radar level value at present (mm)

Reading points: two bytes, either 0001 or 0002 isp optional;
CRC: check code, two bytes;

Slave computer reponding:

Station number (1B) function code (1B) number of bytes read (1B) data (2B) CRC (2B)
T1~T4 01 03 02 XX XX XX XX T1~T4

In which: for any frame beginning and ending with T1~T4, 3~5 quiescent stages should be remained.
Station number (address): one byte “01”,
Function code: one byte “03”,
Data: two bytes, forming 16-bit binary data with the highest byte in the front,
CRC: check code, two bytes.

Rules on CRC code calculation

1. Preset a 16-bit register for hex FFFF (i.e. all for 1). Then name the register CRC register;
2. Perform a bitwise exclusive OR on the first 8-bit data and low bytes of the 16-bit CRC register, and save the results on the CRC register;
3. Check if the lowest byte is 0, and shift the contents of register right one bit if it is 0 (to lower bytes), and refill the highest byte with 0 ; if it is 1, shift the contents of register right one bit if it is 0 (to lower bytes), and refill the highest byte with 0, and then perform a exclusive OR on the CRC register and a polynomial A001 (1010 0000 0000 0001);
4. Repeat step 3, until shift right 8 times, so that the whole 8-bit data are all processed;
5. Repeat step 2 to 4 for processing nest 8-bit data;
6. The CRC register obtained at last is CRC code. When to put CRC results into the information frame, make exchanges of high and low bytes with the lowest one in the front.

Communication protocol example:

Data sent by host: