General discussion • device reports readiness to read but returned no data (device disconnected or multiple access on port?)

Code:

import minimalmodbusimport serialimport timeUSB_PORT = '/dev/ttyUSB0'BAUDRATE = 9600TIMEOUT = 2WRITE_START = 4# One shared instrumentvfd = minimalmodbus.Instrument(USB_PORT, 10)  # Default address, will changevfd.serial.baudrate = BAUDRATEvfd.serial.bytesize = 8vfd.serial.parity = serial.PARITY_NONEvfd.serial.stopbits = 1vfd.serial.timeout = TIMEOUTvfd.mode = minimalmodbus.MODE_RTUvfd.clear_buffers_before_each_transaction = Truevfd.close_port_after_each_call = Truedef set_speed(address, drive_mode, speed):    try:        print(f"Setting speed for address {address}")        vfd.address = address        vfd.serial.close()  # Force close the port before each operation        time.sleep(0.1)  # Give some time before reopening        vfd.serial.open()  # Reopen the port        vfd.write_registers(WRITE_START, [speed, drive_mode])    except Exception as e:        print(f"Error setting speed for address {address}: {e}")def stop_pump(address):    try:        print(f"Stopping pump at address {address}")        vfd.address = address        vfd.serial.close()  # Force close the port before each operation        time.sleep(0.1)  # Give some time before reopening        vfd.serial.open()  # Reopen the port        vfd.write_registers(WRITE_START, [0, 1])  # speed = 0, mode = stop    except Exception as e:        print(f"Error stopping pump at address {address}: {e}")try:    while True:        set_speed(10, 2, 4500)  # SOL VFD        time.sleep(180)        stop_pump(10)        time.sleep(90)except KeyboardInterrupt:    stop_pump(10)    print("Exiting gracefully")