ModbusProxyESP32-P4-ETH

iot-modbus-proxy

ESPHome firmware for a Waveshare ESP32-P4-ETH board acting as a three-port Modbus RTU gateway. The device sits between a Garo GNM3D energy meter, a SolarEdge solar inverter, and a Garo GLB EV charger — translating register maps on the fly so all three devices talk to each other over separate RS-485 buses without any direct wiring between them.

Architecture

                        ┌────────────────────────────────────┐
                        │        ESP32-P4-ETH                │
                        │                                    │
  Bus A  ──────────────►│  MASTER (UART1)                    │
  Garo GNM3D meter      │  polls FC04 INPUT registers        │
  address 0x02          │  every 10 s → caches in sensors    │
                        │                                    │
  Bus B  ◄──────────────│  SERVER 1 (UART2)                  │
  SolarEdge inverter    │  responds to FC03 as WattNode 202  │
  expects WattNode      │  reg 1000–1033, 1100–1181 (FP32_R) │
                        │                                    │
  Bus C  ◄──────────────│  SERVER 2 (UART3)                  │
  Garo GLB charger      │  responds as GNM3D clone           │
  expects GNM3D         │  same addr/type/raw values         │
                        └────────────────────────────────────┘

Each bus is electrically isolated (separate MAX485 transceivers). All communication is Modbus RTU at 9600 bps 8N1.

Data flow

1. Every 10 seconds the master controller on Bus A sends FC04 Read Input Registers to the Garo meter and stores the decoded values in ESPHome sensor entities.
2. When the SolarEdge on Bus B queries a WattNode register, the matching read_lambda pulls the cached sensor value, applies the WattNode unit convention, and returns it as an IEEE 754 float.
3. When the Garo charger on Bus C queries a meter register, the matching read_lambda inverse-scales the cached sensor value back to the raw integer the GNM3D would have returned.

Hardware

GPIO pin assignment

Reserved pins (do not use):

• Ethernet IP101: GPIO28, 29, 30, 31, 34, 35, 49, 50, 51, 52
• I2C ES8311: GPIO7 (SDA), GPIO8 (SCL)
• I2S audio: GPIO9–13
• Speaker enable: GPIO53
• UART0 logger: USB-C console port

MAX485 wiring (identical for all three channels)

ESP32-P4              MAX485               RS-485 bus
──────────────        ─────────────        ──────────
GPIO TX    ──────►  DI  (pin 4)
GPIO RX    ◄──────  RO  (pin 1)
flow_ctrl  ──────►  DE  (pin 3)  ──┐
                     RE  (pin 2) ──┘  (tie together)
                     A   (pin 6)  ──────────── A
                     B   (pin 7)  ──────────── B
                     VCC          ────────────  3.3 V
                     GND          ────────────  GND

The flow_control_pin on each modbus hub is asserted HIGH before transmitting and released LOW after the last byte. No manual GPIO control is needed.

For cable runs longer than 5 m fit a 120 Ω termination resistor between the A and B conductors at each physical end of the segment.

Modbus register maps

Bus A — Garo GNM3D-LP-RS485 (master, FC04 input registers)

All registers are INPUT registers (FC04). 32-bit values use big-word-order (high word at lower address; S_DWORD_R in ESPHome). 16-bit values are signed (S_WORD).

Byte order note: The GNM3D stores INT32 values in big-big endian (both byte and word order are big-endian). This differs from the more common big-little convention. S_DWORD_R in ESPHome reads the high word from the lower register address, which matches the GNM3D native layout.

Bus B — WattNode 202 emulation for SolarEdge (server 1, FC03 holding registers)

The SolarEdge inverter expects a Continental Control Systems WattNode Modbus meter. All registers are 32-bit IEEE 754 floats stored in big word order (high word at lower address; FP32_R in ESPHome), matching the WattNode Modbus specification.

Registers not available from the GNM3D return 0.0.

Block 1000 — Power, voltage, frequency

Block 1100 — Per-phase energy, power factor, reactive/apparent power, currents

Registers 1600–1740 (WattNode configuration and identity blocks) are silently absorbed by server_courtesy_response which returns 0 for any address not explicitly registered.

Unit note: The WattNode register map is inconsistent in power units between the GNM3D source data — total active power (register 1008) is in kW, while per-phase active powers (registers 1010–1014) are in W. This matches the behaviour of the Python reference proxy which writes GNM3D values directly without conversion.

Bus C — Garo GNM3D transparent passthrough (server 2)

The Garo GLB charger is natively designed to communicate with the GNM3D companion meter and queries INPUT register addresses directly. This server mirrors the GNM3D register map by inverse-scaling the cached sensor values back to the raw integers the real meter would return.

All 32-bit registers use S_DWORD_R (big word order) and all 16-bit registers use S_WORD (signed), matching the GNM3D native format.

Address gaps in the GNM3D register map (e.g. 0x003B–0x003F) are handled by server_courtesy_response which returns 0.

Configuration

Substitutions

All pin and address configuration is in the substitutions: block at the top of iot-modbus-proxy.yaml:

substitutions:
  mb_master_tx:  GPIO16   # Bus A TX → MAX485 DI
  mb_master_rx:  GPIO14   # Bus A RX ← MAX485 RO
  mb_master_de:  GPIO15   # Bus A DE/RE

  mb_slave1_tx:  GPIO17   # Bus B TX
  mb_slave1_rx:  GPIO19   # Bus B RX
  mb_slave1_de:  GPIO18   # Bus B DE/RE

  mb_slave2_tx:  GPIO22   # Bus C TX
  mb_slave2_rx:  GPIO20   # Bus C RX
  mb_slave2_de:  GPIO21   # Bus C DE/RE

  slave1_addr:   "2"      # SolarEdge expects WattNode at address 2
  slave2_addr:   "2"      # Garo charger expects meter at address 2

  master_poll_interval: "5s"

Secrets

Create a secrets.yaml file alongside the main YAML:

modbus_proxy_api_encryption_key: "your_32_byte_base64_key_here"
modbus_proxy_ota_password:       "your_ota_password_here"

Build and flash

Requires ESPHome >= 2025.1.0 (ESP32-P4 support).

# Compile (no device needed — checks YAML validity)
esphome compile iot-modbus-proxy.yaml

# Compile and upload via USB
esphome run iot-modbus-proxy.yaml

# OTA update (device must be on the network)
esphome run iot-modbus-proxy.yaml --device <IP_ADDRESS>

# View serial logs
esphome logs iot-modbus-proxy.yaml

The board uses USB-C for both power and serial flashing. UART0 is reserved for the ESPHome logger; do not reassign it.

Known limitations and caveats

Floating-point precision for large energy values: The Bus C passthrough inverse-scales sensor floats back to raw integers. IEEE 754 single precision can represent integers exactly up to 2²⁴ = 16,777,216. For accumulated energy this means values above ~1,677,721 kWh (~1.7 GWh) may lose the last digit of precision. In practice this is not a concern for residential or small commercial installations.

Missing per-phase fields: The GNM3D does not expose per-phase reactive/apparent instantaneous power or per-phase export energy. Those WattNode registers return 0.0. The SolarEdge typically does not require these for basic energy yield monitoring.

Polling latency: Sensor values are refreshed every 5 seconds. The SolarEdge and Garo charger will read values that are up to 5 seconds old. Reduce master_poll_interval if more frequent updates are needed, keeping in mind the increased RS-485 bus load.