ESP32 Modbus Slave Simulator

Why simulate a Modbus slave on an ESP32?

The ESP32's built-in Wi-Fi (and optional Ethernet via an add-on PHY) makes it a natural fit for simulating a networked Modbus TCP field device — no laptop, no extra network adapter, just power and a Wi-Fi connection. That makes it popular for IIoT prototyping: simulating a sensor node, edge gateway, or remote I/O unit that a PLC or SCADA system polls over the network, at a fraction of the cost and footprint of a full field device.

Two library options

Pick modbus-esp8266 if your project already lives in Arduino/PlatformIO. Pick esp-modbus if you're building a native ESP-IDF project and want the component Espressif maintains directly.

Modbus TCP slave on ESP32 (Arduino, modbus-esp8266)

No extra hardware is needed beyond Wi-Fi. A minimal TCP slave looks like this:

#include <WiFi.h>
#include <ModbusIP_ESP8266.h>

ModbusIP mb;
const int HREG_START = 0;

void setup() {
  WiFi.begin("your-ssid", "your-password");
  while (WiFi.status() != WL_CONNECTED) delay(500);

  mb.server();                       // start Modbus TCP slave on port 502
  mb.addHreg(HREG_START, 0, 100);    // 100 holding registers, initial value 0
}

void loop() {
  mb.task();
}

Point any Modbus master (PLC, SCADA, or a desktop simulator) at the ESP32's IP address on port 502 to read/write the simulated holding registers.

Modbus RTU slave on ESP32 (RS485 wiring)

The ESP32's UART pins output 3.3V TTL, not RS485 levels, so Modbus RTU needs a MAX485 (or similar) RS485 transceiver module wired between a UART pair and the bus:

1. Wire the transceiver's RO/DI pins to an ESP32 UART (e.g. UART2 on GPIO16/17), and its DE/RE pins to a spare GPIO for direction control
2. Wire A/B to the RS485 bus, matching polarity with the master
3. Configure the library's serial server with the matching baud rate, parity, and unit ID your master expects
4. Toggle DE/RE around each transmit in your loop (or use a library helper that does this for you) and confirm the master can read registers

ESP32 vs a desktop Modbus simulator

When you don't need an ESP32 at all

If your goal is simply to have a Modbus slave to validate PLC logic, HMI tags, or SCADA polling — without needing the ESP32's wireless/edge-device advantages — a desktop Modbus simulator gives you the same TCP and RTU slave behavior with a visual register editor, no firmware flashing, and no separate board to keep powered and connected.

Related pages

Raspberry Pi as a Modbus Slave Simulator · Arduino Modbus RTU Slave — Setup and Testing Guide · Modbus to MQTT Bridge — Testing Before Deployment · Modbus RTU Simulator · Modbus TCP Simulator

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