• Serial connection streaming

    dotvav • 02/22/2026 at 13:47 • 0 comments

    Today I stream the serial connection on the wifi.

    I am using an ESP32 C2 Mini that was laying around.


    I flash ESPHome on it with the following configuration.

    yutampo_bridge.yaml

    esphome:
  name: yutampo-bridge
  includes:
    - invert_uart.h
  on_boot:
    priority: 200 
    then:
      - lambda: force_uart_inversion();

esp32:
  board: lolin_s2_mini
  framework:
    type: esp-idf

external_components:
  - source: github://oxan/esphome-stream-server

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

api:
ota:
  - platform: esphome

logger:
  level: DEBUG
  baud_rate: 0

uart:
  id: uart_bus
  tx_pin: GPIO2
  rx_pin: GPIO3
  baud_rate: 9600
  data_bits: 8
  parity: EVEN
  stop_bits: 1

stream_server:
  uart_id: uart_bus
  port: 8888

    invert_uart.h

    #include "driver/uart.h"

void force_uart_inversion() {
    // The ESP32-S2 has two UARTs. UART0 is the logger, UART1 is our heater bus.
    // This commands the silicon to physically invert the RX and TX pins.
    uart_set_line_inverse(UART_NUM_1, UART_SIGNAL_RXD_INV | UART_SIGNAL_TXD_INV);
}

    Plugged into and powered by the Yutampo connection, I connect via Telnet and get the following:

     Telnet  Connecting to 192.168.1.243
 Telnet  Connected
ANS OK

ANS OK


ANS OK


ANS OK


ANS OK


ANS OK


ANS OK


ANS OK


>

     I tried passing some commands but got no responses.

  • First steps on the serial protocol

    dotvav • 02/21/2026 at 14:12 • 0 comments

    My home has Hitach heat pumps that are used for air-air AC and domestic hot water. For several years, I have used Hitachi's proprietary cloud gateways and app to control the AC units. In 2019 I partially reversed engineered the Hi-Kumo cloud protocol and built a custom MQTT Home Assistant integration: https://github.com/dotvav/aasivak. In 2023 I teamed up with the maintener of the Hi-Kumo integration in Home Assistant to add support for the air-air heat pumps. I was always annoyed to be forced to use the Hi-Kumo cloud, and in 2025 I found that someone had created a fully local ESPHome integration: https://github.com/lumixen/esphome-hlink-ac.

    When the water heater was installed at home, I was told there was a domotic integration available, but it was very expensive and required a separate cloud gateway. I decided that it was a nice to have, but I did not need it. Now that I have found how "easy" it is with the air-air AC, I want to figure out if it is also possible for the water heater.

    I asked Lumixen if they had any guidance or advice and they directed me to Florian's project: https://hackaday.io/project/168959-let-me-control-you-hitachi-air-conditioner, who had reverse engineered the serial connection protocol of the air-air AC.

    Because the Yutampo water heater has the same 6-pin "JST PH 2 mm" connector labeled "CN7", I thought it would be very similar. I tested the different leads with a voltmeter and they seemed to match:

    PinLevelSignal
    1
    		12V+VCC
    25VTX
    3--
    45VRX
    50VGND
    65V?

    I tried pluging in one of my Lumixen air-air AC ESPHome modules, but it wouldn't work at all. One of the differences is the AC units can receive read and write commands, and this is signaled by the pin 6 being low (shorted with GND). On the Yutampo, it is high by default, and shorting it with GND provokes a VCC collapse from 12 to 2.4V, so I stopped trying that.

    In order to run tests without having to open the motherboard's enclosure, I recycled an old ethernet cable and soldered a 6-pin "JST PH 2 mm" connector on both ends. The Hitachi end is close enough to that form factor, and the connector needs only a little bit of filing to fit in it.

    With the cable installed and easy to acess, I decided to try some signal analysis. I flashed an ESP8266 with https://github.com/hepter/web-logic-analyzer.

    I powered with an USB power bank and plugged pins 2, 4 and 5, turned off the water heater, started the measure, and turned on the water heater. When done, I loaded the data in PulseView, played a bit with the UART analyser and found the following:

    • Type: UART
    • Speed: 9600 bauds.
    • Configuration: 8 bits data, no parity, no stop (8N0 observed).
    • Format: ASCII.
    • Observed behavior: Emitted data every 3000ms.
    • Message: `ANS OK\r\n` (Hex: `41 4E 53 20 4F 4B 0D 0A`).

    The same "ANS OK" message is being emitted by the motherboard every 3s.

    My next step will be trying to send messages and see if it accepts the same message formats as the AC units and, if not, try my luck with some standard messages like "?", "help", "stat", "info", "ver"...

    If nothing works, I may try to procure myself some working supported Hitachi devices and spy on the protocol, but these things are not particularly easy to find or cheap. I'd need an "ATW-HCD-01" adapter (that converts the 6 pins CN7 H-Link connection into a 2 wires one) and an "ATW-TAG-02" (that connects this into some cloud solution).