Smart Meter / Switch KWS-303L with RS-485 Modus

I found this switch on Aliexpress. It’s quite interesting, but it has lack of documentation.

All technical details also available at github.

It has following features:

• Frequency meter
• Voltage meter
• Consumed energy meter
• Current meter
• Temperature meter (via thermal couple in NTC connector)
• RS-485 connectivity (modbus) connector
• Active power meter
• Power factor meter
• On/Off switch
• Over/Under voltage protection
• Current Limit
• Max current 40A

Inside I found quite accurate soldering, thick busbars and AT32F421 32-bit ARM Cortext-M4 MCU.

And there is no any description how to communication with RS-485. Even seller don’t know how to use it. In the settings menu of this device I’v assigned modbus address 2 and start testing.

I suspected that it should has a number of holing registers and to identify the list of registers I wrote simple python script, that just scan all registers from device:

from pymodbus import FramerType
from pymodbus.client import ModbusSerialClient

client = ModbusSerialClient(port="/dev/tty.usbserial-A5069RR4", baudrate=9600, parity='E', 
        stopbits=1, timeout=0.1, retries=1, framer=FramerType.RTU)
client.connect()


for i in range(1,100):
  try:
    rr = client.read_holding_registers(i, count=1, slave=2)
    v = client.convert_from_registers(rr.registers, word_order='big', data_type=client.DATATYPE.INT16)
    print("%d | %d" % (i, v))

  except Exception as e:
    print(">>ERROR: %s" % (str(e)))
    pass

client.close()

But before I had to found correct baud rates, stop bits and parity. In reality most of the cheap devices use 9600 as baud rate. So there were no many combination I had to check …

So the configuration is following:

Here the list of holding registers I found:

Tags: baud-rateholding-registerhome-assistanthome-automationinput-registerkws-303kws-303lkws303kws303lmetermodbusmodbus-rtuparitypinoutprotocolregisterrs-485rs485rtusmart-homesmart-meterstop-bitsswitch