Beginners • A02YYUW Waterproof sensor, UART works, but need to write data to read data

Code:

from machine import UART, Pinfrom os import unamefrom sys import implementationimport time# A02YYUW Waterproof sensor, UART-version (mine is: A0221A)# connected with RS-485## Notes:# * Pico has two UARTs: uart(0) and uart(1)# * Define uart1 pins for ultrasonic, uart0 typical for REPL so don't use uart0## * 4 Sensor wires: 1) Red=Vcc, 2)Blk=Gnd,#                   3) Yellow=rx_sensor=pico_tx1(GP20)#                   4) White =tx_sensor=pico_rx1(GP21)## * https://dronebotworkshop.com/waterproof-ultrasonic/  (note diff wire colors on sensor)#   Sensor's RX pin controls mode. When held HIGH or not connected (it is internally pulled up),#   it will operate every 300ms. If the RX pin is held LOW then the data is output every 100ms.#   Pico's UART RX pin to receive the signal/values from the senor's TX pin##   BUT, only way it works for me is to write a byte each time to get sensor to respond## DEBUG NEEDED: "with Rx on the sensor floating, it will output data ever 300ms" -- CANT GET THIS WORKING## by bradcardebug =Falseuart1 = machine.UART(1, 9600, tx=20, rx=21)# expect: UART(1, baudrate=9600, bits=8, parity=None, stop=1, tx=20, rx=21, txbuf=256, rxbuf=256, timeout=0, timeout_char=2, invert=None, irq=0)print(uart1)# Function to calculate checksum, make sure only 1 byte returneddef calculate_checksum(data_buffer):    return (data_buffer[0] + data_buffer[1] + data_buffer[2]) & 0x00ffdef ultrasonic_distance_uart():    """    :returns: distance in cm (type: float), The sensor returns mm in integer    """    # REQUIRED write !!!!   to get uart1 to send data    uart1.write(b'\xff')        if uart1.any():           # Read the first byte to check for the packet header (0xFF)        if uart1.read(1) == b'\xff':            # Create an array to store the data buffer            data_buffer = bytearray(4)            data_buffer[0] = 0xff            # Read the next 3 bytes            data_buffer[1:4] = uart1.read(3)            if debug: print(f"data_buffer = {data_buffer}")            # Compute checksum            checksum = calculate_checksum(data_buffer)            if debug: print(f"checksum={checksum:x}")            # Verify if the checksum matches the last byte in the packet            if data_buffer[3] == checksum:                # Calculate distance in mm from the data bytes                distance = (data_buffer[1] << 8) + data_buffer[2]                if debug: print(f"distance={distance} mm\n")                return distance/10.0    return None# startup codeprint("Starting...")print("====================================")print(implementation[0], uname()[3],      "\nrun on", uname()[4])print("====================================")# A02YYUW seems to need one call before use, then wait 500ms, otherwise get errors the first time_ = ultrasonic_distance_uart()time.sleep(.5)# Main loopwhile True:    cm = ultrasonic_distance_uart()    if cm:        print(f"Distance= {cm:.1f} cm")    else:        print("ERROR A02YYUW: No valid data received.")        time.sleep(1)  # arbitrary One second between measurements