Chapter 188: EtherCAT Slave Implementation

Chapter Objectives

By the end of this chapter, you will be able to:

• Understand the fundamental principles of EtherCAT (Ethernet for Control Automation Technology).
• Describe the “processing on the fly” concept and its impact on performance.
• Explain the EtherCAT network topology, telegram structure, and addressing.
• Understand the role and importance of Distributed Clocks (DC) for synchronization.
• Outline the EtherCAT State Machine (ESM) and its different states.
• Recognize the purpose and structure of EtherCAT Slave Information (ESI) files.
• Identify the critical hardware requirement for EtherCAT slaves: the EtherCAT Slave Controller (ESC).
• Discuss conceptual approaches for an ESP32 to interface with an ESC and manage application logic for an EtherCAT slave.
• Appreciate the differences among ESP32 variants concerning their suitability as host controllers for ESCs.
• Be aware of common challenges and troubleshooting techniques in EtherCAT slave development.

Introduction

In the realm of industrial automation, the demand for higher performance, greater precision, and real-time synchronization has driven the evolution of Industrial Ethernet protocols. EtherCAT, standing for Ethernet for Control Automation Technology, is a high-performance, deterministic Ethernet-based fieldbus system developed by Beckhoff Automation. It is renowned for its exceptional speed, efficiency, and precise synchronization capabilities, making it a preferred choice for demanding applications like motion control, robotics, and high-speed data acquisition.

EtherCAT achieves its performance through a unique principle: “processing on the fly.” Instead of traditional store-and-forward Ethernet frame handling by each node, EtherCAT telegrams pass through slave devices with minimal delay, as data is extracted and inserted on the fly by specialized hardware. This results in extremely short cycle times (down to microseconds) and low communication jitter.

While implementing a full EtherCAT master or slave stack purely in software on a general-purpose microcontroller like the ESP32 is exceptionally challenging (and generally not feasible for the slave due to hardware requirements), the ESP32 can serve as an intelligent host controller for a dedicated EtherCAT Slave Controller (ESC) ASIC or FPGA. This chapter will introduce the core concepts of EtherCAT and explore how an ESP32, using ESP-IDF v5.x, can be integrated with an ESC to create an EtherCAT slave device.

Theory

EtherCAT Core Principles

EtherCAT’s design philosophy sets it apart from many other Industrial Ethernet solutions.

• Processing on the Fly: This is the cornerstone of EtherCAT’s performance.
  • An Ethernet frame (EtherCAT telegram) sent by the master traverses through all connected slave devices.
  • Each slave device has a dedicated EtherCAT Slave Controller (ESC). As the telegram passes through the ESC, the ESC reads the data addressed to it and inserts its own data into the frame as it passes through.
  • The delay per slave is typically nanoseconds. The frame is only slightly delayed by each slave, primarily due to signal propagation.
  • The last slave in a segment (or a logical segment end) reflects the frame back to the master.
  • This means a single Ethernet frame can carry data for many slaves, both input and output, significantly improving bandwidth utilization and reducing protocol overhead.

%%{ init: { 'theme': 'base', 'themeVariables': { 'fontFamily': 'Open Sans, sans-serif' } } }%%
graph TD
    subgraph EtherCAT Network Cycle
        direction TB
        Master[("<b>EtherCAT Master</b><br>Sends Telegram")]
        Slave1[("<b>Slave 1 (ESC)</b><br>Reads/Writes Data<br><i>nanoseconds delay</i>")]
        Slave2[("<b>Slave 2 (ESC)</b><br>Reads/Writes Data<br><i>nanoseconds delay</i>")]
        SlaveN[("<b>Slave N (ESC)</b><br>Reads/Writes Data<br><i>nanoseconds delay</i>")]
        ReturnPath{{"Telegram<br>Returns"}}

        Master -- "Telegram (Frame with EtherType 0x88A4)" --> Slave1
        Slave1 -- " " --> Slave2
        Slave2 -- " " --> SlaveN
        SlaveN -- "Frame looped back" --> ReturnPath
        ReturnPath -- "Processed Telegram<br>with all Slave Data & WKC" --> Master
    end

    %% Styling
    style Master fill:#EDE9FE,stroke:#5B21B6,stroke-width:2px,color:#5B21B6
    style Slave1 fill:#DBEAFE,stroke:#2563EB,stroke-width:1px,color:#1E40AF
    style Slave2 fill:#DBEAFE,stroke:#2563EB,stroke-width:1px,color:#1E40AF
    style SlaveN fill:#DBEAFE,stroke:#2563EB,stroke-width:1px,color:#1E40AF
    style ReturnPath fill:#D1FAE5,stroke:#059669,stroke-width:2px,color:#065F46

• Network Topology:
  • EtherCAT supports flexible topologies: line, tree, star, or combinations.
  • Logically, EtherCAT always operates as a bus. Even if physically wired as a star (using EtherCAT junction devices with multiple ports), the telegrams effectively traverse a logical line.
  • Each ESC typically has two (or more for junction devices) RJ45 ports (Port 0, Port 1, etc.). The frame enters one port and exits another. If a port is not connected, the ESC automatically closes the logical loop at that point and returns the frame.
• EtherCAT Telegram Structure:
  • EtherCAT uses standard IEEE 802.3 Ethernet frames with a specific EtherType: 0x88A4.
  • Within the Ethernet payload, one or more EtherCAT datagrams are encapsulated.
  • Each datagram contains:
    • Header: Command (e.g., APRD – Auto-increment Physical Read, FPWR – Configured Address Physical Write), Index (address within the slave), Length of data.
    • Data: The actual process data or control/status information.
    • Working Counter (WKC): A crucial field. Each slave that successfully processes a datagram addressed to it increments the WKC. The master uses the final WKC value to verify that all addressed slaves correctly handled their part of the datagram.

• Addressing:
  • Auto-Increment Addressing: Slaves are addressed by their physical position in the network ring. The first slave is address 0, the second is -1, the third is -2, and so on (relative to the master’s perspective during configuration).
  • Configured Address (Station Alias): During initialization, the master assigns a unique 16-bit fixed address (Station Alias) to each slave. This address is then used for subsequent communication.
  • Logical Addressing (Process Data): The master configures a logical process data image. Data for multiple slaves can be mapped into this single image, allowing the master to read/write all relevant process data with a single EtherCAT frame containing multiple datagrams.

Distributed Clocks (DC)

For applications requiring high-precision synchronization (e.g., coordinated motion control), EtherCAT offers Distributed Clocks.

• Principle: Each ESC contains a high-precision local clock (typically 64-bit).
• Synchronization: The master sends a special EtherCAT datagram (e.g., FRMW – Fixed Address Read Multiple Write) to the reference clock slave (usually the first DC-capable slave in the network). This slave writes its current time into the frame. As the frame passes through subsequent DC slaves, they read this reference time and also the time written by the previous slave, allowing them to calculate propagation delays.
• Offset and Drift Compensation: Each slave calculates the offset between its local clock and the reference clock and compensates for it. Drift compensation is also performed.
• Result: All DC-capable slaves in the network can be synchronized with sub-microsecond precision. This enables precisely timed I/O operations and coordinated actions across multiple devices.
• Sync Signals (SYNC0, SYNC1): DC-capable ESCs can generate precisely timed interrupt signals (SYNC0, SYNC1) based on their synchronized clock, triggering actions in the slave’s application (e.g., ADC sampling, actuator output).

EtherCAT State Machine (ESM)

Every EtherCAT slave device must implement the EtherCAT State Machine. The master controls the state transitions of its slaves.

• Init (Initialization):
  • Default state after power-on or reset.
  • Basic communication (register access) is possible, but no process data exchange or mailbox communication.
  • The ESC is initialized.
• Pre-Operational (PreOp):
  • Mailbox communication is possible (for SDO access, configuration).
  • Process data exchange is not yet active.
  • Slaves check their configuration parameters received from the master.
  • Distributed Clocks can be initialized and synchronized.
• Safe-Operational (SafeOp):
  • Input process data is cyclically read by the master.
  • Output process data is accepted by the slave, but outputs are typically kept in a safe state (e.g., motors off).
  • This state allows the master to verify the system configuration and inputs before enabling outputs.
• Operational (Op):
  • Full process data exchange (inputs and outputs) is active.
  • The device is fully operational and participating in the control process.
  • Outputs are controlled by the master via process data.
• Bootstrap (Boot):
  • An optional state for firmware updates via EtherCAT (FoE – File access over EtherCAT).

%%{ init: { 'theme': 'base', 'themeVariables': { 'fontFamily': 'Open Sans, sans-serif' } } }%%
stateDiagram-v2
    direction TB
    state "<b>Init</b><br>Basic Register Access" as Init
    state "<b>Pre-Operational</b><br>(PreOp)<br>Mailbox Communication" as PreOp
    state "<b>Safe-Operational</b><br>(SafeOp)<br>Inputs Read, Outputs Safe" as SafeOp
    state "<b>Operational</b><br>(Op)<br>Full I/O Exchange" as Op
    state "<b>Bootstrap</b><br>(Boot)<br><i>Firmware Update (FoE)</i>" as Boot

    [*] --> Init : Power On / Reset
    Init --> PreOp : Master Request (I -> P)
    PreOp --> Init : Master Request (P -> I)

    PreOp --> SafeOp : Master Request (P -> S)<br><i>Valid config received</i>
    SafeOp --> PreOp : Master Request (S -> P)

    SafeOp --> Op : Master Request (S -> O)<br><i>Outputs enabled</i>
    Op --> SafeOp : Master Request (O -> S)<br><i>or Error</i>

    Op --> PreOp : Master Request (O -> P)
    Op --> Init : Master Request (O -> I)

    Init --> Boot : Master Request (I -> B)
    Boot --> Init : Master Request (B -> I)

Object Dictionary (OD) and Data Exchange

• Object Dictionary (OD):
  • Similar to CANopen, EtherCAT slaves can have an Object Dictionary.
  • It’s a structured collection of all data items accessible in the slave, including device parameters, configuration settings, and process data mappings.
  • Each entry is identified by a 16-bit Index and an 8-bit SubIndex.
  • The OD provides a standardized way to access device information.
• Process Data Objects (PDOs):
  • Used for fast, cyclic exchange of real-time process data (inputs and outputs).
  • PDOs map data from/to the slave’s application to/from the EtherCAT process data image.
  • Configured by the master during startup based on the ESI file and master configuration.
  • Transmitted within the cyclic EtherCAT datagrams.
• Service Data Objects (SDOs):
  • Used for acyclic access to entries in the Object Dictionary (e.g., reading/writing device parameters).
  • Typically communicated via the EtherCAT mailbox mechanism (which itself uses EtherCAT datagrams).
  • Slower than PDO communication, used for configuration and diagnostics. EtherCAT offers different mailbox protocols like CoE (CANopen over EtherCAT), SoE (Servo drive over EtherCAT), EoE (Ethernet over EtherCAT). CoE is very common.

EtherCAT Slave Information (ESI) Files

• Purpose: The ESI file is an XML-based device description file. It provides the EtherCAT master (and its engineering tool) with all necessary information about the slave device.
• Content:
  • Vendor information (Name, ID).
  • Device information (Product Code, Revision Number, Device Name).
  • EtherCAT State Machine behavior.
  • Process Data Items (PDO mapping options).
  • Object Dictionary entries (if CoE is supported).
  • Mailbox configuration.
  • Distributed Clocks parameters.
  • Physical port descriptions.
• Format: Standardized XML schema defined by the EtherCAT Technology Group (ETG).
• Naming Convention: Typically VendorName_DeviceName_Revision.xml or similar.
• Role: The master’s configuration tool imports the ESI file to understand the slave’s capabilities, configure it, and set up the process data exchange.

The Crucial Role of the EtherCAT Slave Controller (ESC)

Unlike some fieldbuses that can be implemented largely in software on a general-purpose MCU, EtherCAT slaves require dedicated hardware for the “processing on the fly” mechanism. This hardware is the EtherCAT Slave Controller (ESC).

• Functionality:
  • Handles the low-level EtherCAT protocol (frame reception, parsing, on-the-fly data extraction/insertion, frame forwarding/loopback).
  • Manages the EtherCAT State Machine.
  • Implements Process Data Interface (PDI) for exchanging data with the host microcontroller (the ESP32 in our case). Common PDIs include SPI, parallel bus (e.g., FSMC-like), or I/O signals.
  • Contains memory (DPRAM – Dual-Ported RAM or Process Data RAM) accessible by both the EtherCAT master (via the network) and the host MCU (via PDI).
  • Implements Distributed Clocks (if DC-capable).
  • Generates interrupts to the host MCU for events like state changes, new process data, SYNC signals.
• Examples of ESCs:
  • Beckhoff ET1100, ET1200 (ASICs).
  • FPGA IP cores from various vendors (e.g., Beckhoff, Hilscher, TI).
  • Microcontrollers with integrated ESCs (e.g., TI Sitara AMxxxx series, Renesas R-IN32M series).

An ESP32 cannot act as an EtherCAT slave without an external ESC. The ESP32’s built-in Ethernet MAC is for standard Ethernet communication; it does not have the specialized hardware logic for EtherCAT’s on-the-fly processing. The ESP32’s role is to act as the host application processor for the ESC.

Practical Examples

This section will focus on the conceptual interaction between an ESP32 and an external EtherCAT Slave Controller (ESC). We’ll assume an ESC (like a Beckhoff ET1100 or similar) is connected to the ESP32 via SPI. The actual EtherCAT slave stack logic (which would run on the ESP32 and manage the ESC) is highly complex, so we will illustrate the types of interactions.

Hardware Setup

1. ESP32 Development Board.
2. EtherCAT Slave Controller (ESC) Module/Board: A board featuring an ESC (e.g., ET1100) with an SPI interface for the host MCU. This module would also have two RJ45 ports for the EtherCAT network.
   • Example: A custom board or a development kit for an ESC.
3. Connections (ESP32 to ESC via SPI):
   • ESP32 SPI MOSI -> ESC SPI MOSI
   • ESP32 SPI MISO <- ESC SPI MISO
   • ESP32 SPI SCLK -> ESC SPI SCLK
   • ESP32 GPIO (CS) -> ESC SPI Chip Select
   • ESP32 GPIO (IRQ) <- ESC Interrupt Output
   • Power and Ground connections.
4. EtherCAT Master: A PC with EtherCAT master software (e.g., TwinCAT, CODESYS with EtherCAT master, acontis ec-master) or a dedicated EtherCAT master controller.
5. EtherCAT Cables: Standard Ethernet cables (CAT5e or better).

Software: ESP32 Interfacing with an ESC (Conceptual)

We’ll outline the ESP32 application structure, assuming an EtherCAT slave stack library (e.g., a port of SOEM/Simple Open EtherCAT Master, or a dedicated slave stack like SSC Tool generated code from Beckhoff, or a commercial stack) is available for the ESP32 to manage the ESC.

Project Setup in VS Code

1. Create a new ESP-IDF v5.x project (e.g., esp32_ethercat_slave).
2. Integrate the chosen EtherCAT slave stack library as a component. This step is highly dependent on the specific stack. For example, Beckhoff’s SSC (Slave Stack Code) tool can generate C code based on an ESI file, which would then be compiled for the ESP32.

ESI File Creation (Conceptual Snippet)

An ESI file describes the slave device to the master. This is typically created using an ESI editor tool.

<!-- Simplified ESI File Snippet for a hypothetical ESP32-hosted ESC -->
<EtherCATInfo Version="1.6" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="EtherCATInfo.xsd">
  <Vendor>
    <Id>0x00000E5F</Id> <!-- Hypothetical Vendor ID for "Espressif Systems EDU" -->
    <Name>Espressif Systems EDU</Name>
  </Vendor>
  <Descriptions>
    <Groups>
      <Group>
        <Type>General</Type>
        <Name LcId="1033">ESP32 Hosted EtherCAT Slave</Name>
      </Group>
    </Groups>
    <Devices>
      <Device Physics="YY"> <!-- YY = RJ45 -->
        <Type ProductCode="#x00010001" RevisionNo="#x00010000">ESP32_ECAT_DEMO</Type>
        <Name LcId="1033">ESP32 EtherCAT Demo Slave</Name>
        <GroupType>General</GroupType>
        <Sm MinSize="0" MaxSize="128" DefaultSize="8" StartAddress="0x1000" ControlByte="0x0026" Enable="1" Type="MailboxWrite"/> <!-- SyncManager 0: Mailbox Out -->
        <Sm MinSize="0" MaxSize="128" DefaultSize="8" StartAddress="0x1080" ControlByte="0x0022" Enable="1" Type="MailboxRead"/>  <!-- SyncManager 1: Mailbox In -->
        <Sm MinSize="0" MaxSize="32" DefaultSize="2" StartAddress="0x1100" ControlByte="0x0024" Enable="1" Type="Outputs"/>      <!-- SyncManager 2: Process Data Outputs -->
        <Sm MinSize="0" MaxSize="32" DefaultSize="2" StartAddress="0x1120" ControlByte="0x0020" Enable="1" Type="Inputs"/>       <!-- SyncManager 3: Process Data Inputs -->
        <Pdo> <!-- Example: 2 Bytes Input, 2 Bytes Output -->
          <TxPdo Sm="3">
            <Index>#x1A00</Index>
            <Name>Inputs_2Bytes</Name>
            <Entry>
              <Index>#x7000</Index> <SubIndex>1</SubIndex> <BitLen>8</BitLen> <Name>InputByte1</Name> <DataType>USINT</DataType>
            </Entry>
            <Entry>
              <Index>#x7000</Index> <SubIndex>2</SubIndex> <BitLen>8</BitLen> <Name>InputByte2</Name> <DataType>USINT</DataType>
            </Entry>
          </TxPdo>
          <RxPdo Sm="2">
            <Index>#x1600</Index>
            <Name>Outputs_2Bytes</Name>
            <Entry>
              <Index>#x6000</Index> <SubIndex>1</SubIndex> <BitLen>8</BitLen> <Name>OutputByte1</Name> <DataType>USINT</DataType>
            </Entry>
            <Entry>
              <Index>#x6000</Index> <SubIndex>2</SubIndex> <BitLen>8</BitLen> <Name>OutputByte2</Name> <DataType>USINT</DataType>
            </Entry>
          </RxPdo>
        </Pdo>
        <Mailbox>
          <CoE SdoInfo="true" PdoAssign="false" PdoConfig="false" CompleteAccess="false" SegmentedSdo="true" />
        </Mailbox>
        <!-- DC (Distributed Clocks) Configuration if supported -->
        <Dc>
            <OpMode Name="DC_SYNC0" AssignActivate="#x300">DC</OpMode>
            <SyncUnit AssignActivate="#x300" CycleTimeFactor="1" CycleTime0="1000000" ShiftTime0="0">SYNC0</SyncUnit>
        </Dc>
      </Device>
    </Devices>
  </Descriptions>
</EtherCATInfo>

Warning: This ESI snippet is highly simplified. Real ESI files are complex and generated by specialized tools based on the slave’s hardware (ESC type) and software capabilities. Beckhoff provides an “SSC Tool” (Slave Stack Code) that can generate C code for the slave application and a corresponding ESI file.

Main Application Code (main/main.c)

This code focuses on initializing SPI for ESC communication and simulating interaction with an EtherCAT slave stack.

#include <stdio.h>
#include <string.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "esp_system.h"
#include "esp_log.h"
#include "driver/spi_master.h"
#include "driver/gpio.h"

// --- Hypothetical EtherCAT Slave Stack and ESC Interface ---
// These would be part of your chosen EtherCAT stack library (e.g., from SSC Tool, SOEM port)
// #include "ecat_slave_stack.h" // Main stack header
// #include "esc_if_spi.h"       // ESC SPI driver header

static const char *TAG = "ECAT_APP";

// SPI Configuration for ESC (Example for VSPI bus)
#define ESC_SPI_HOST    SPI3_HOST // HSPI_HOST or VSPI_HOST (SPI2_HOST or SPI3_HOST for IDF v4.x+)
#define PIN_NUM_MOSI    GPIO_NUM_23
#define PIN_NUM_MISO    GPIO_NUM_19
#define PIN_NUM_CLK     GPIO_NUM_18
#define PIN_NUM_CS      GPIO_NUM_5
#define PIN_NUM_IRQ     GPIO_NUM_4  // ESC Interrupt pin

spi_device_handle_t g_esc_spi_handle;

// Application Process Data (example: 2 bytes IN, 2 bytes OUT)
// These variables would be mapped by the EtherCAT stack to the ESC's DPRAM
// based on PDO configuration in the ESI file.
typedef struct {
    uint8_t input_byte1;
    uint8_t input_byte2;
} app_inputs_t;

typedef struct {
    uint8_t output_byte1;
    uint8_t output_byte2;
} app_outputs_t;

app_inputs_t g_app_inputs;
app_outputs_t g_app_outputs;


// --- Conceptual EtherCAT Stack Callbacks / Functions ---
/**
 * @brief Called by the EtherCAT stack to copy outputs from ESC DPRAM to application variables.
 */
void app_read_outputs_from_dpram(void) {
    // This function would be provided by the stack, or the stack updates g_app_outputs directly.
    // Example: ecat_stack_get_output_image(&g_app_outputs, sizeof(g_app_outputs));
    // For this conceptual example, we'll log when it *would* be called.
    // ESP_LOGD(TAG, "Stack indicates new output data: Out1=0x%02X, Out2=0x%02X", g_app_outputs.output_byte1, g_app_outputs.output_byte2);
}

/**
 * @brief Called by the EtherCAT stack to copy inputs from application variables to ESC DPRAM.
 */
void app_write_inputs_to_dpram(void) {
    // This function would be provided by the stack, or the stack reads g_app_inputs directly.
    // Example: ecat_stack_set_input_image(&g_app_inputs, sizeof(g_app_inputs));
    // For this conceptual example, we'll log when it *would* be called.
    // ESP_LOGD(TAG, "Stack requests input data: In1=0x%02X, In2=0x%02X", g_app_inputs.input_byte1, g_app_inputs.input_byte2);
}

/**
 * @brief ESC Interrupt Service Routine (ISR) handler or task notified by ISR
 */
static void IRAM_ATTR esc_interrupt_handler(void* arg) {
    // Notify the main EtherCAT stack task that an ESC event occurred.
    // This is highly stack-dependent. Usually involves a semaphore or queue.
    // BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    // xSemaphoreGiveFromISR(g_esc_event_semaphore, &xHigherPriorityTaskWoken);
    // if (xHigherPriorityTaskWoken) {
    //     portYIELD_FROM_ISR();
    // }
}


static void initialize_esc_spi(void) {
    spi_bus_config_t buscfg = {
        .mosi_io_num = PIN_NUM_MOSI,
        .miso_io_num = PIN_NUM_MISO,
        .sclk_io_num = PIN_NUM_CLK,
        .quadwp_io_num = -1,
        .quadhd_io_num = -1,
        .max_transfer_sz = 4094 // Max transfer size in bytes
    };
    spi_device_interface_config_t devcfg = {
        .clock_speed_hz = 10 * 1000 * 1000, // Clock out at 10 MHz (ESC dependent)
        .mode = 0,                          // SPI mode 0 (CPOL=0, CPHA=0)
        .spics_io_num = PIN_NUM_CS,         // CS pin
        .queue_size = 7,                    // We want to be able to queue 7 transactions at a time
        //.pre_cb=lcd_spi_pre_transfer_callback,  //Specify pre-transfer callback to handle D/C line
    };
    //Initialize the SPI bus
    esp_err_t ret = spi_bus_initialize(ESC_SPI_HOST, &buscfg, SPI_DMA_CH_AUTO);
    ESP_ERROR_CHECK(ret);
    //Attach the ESC to the SPI bus
    ret = spi_bus_add_device(ESC_SPI_HOST, &devcfg, &g_esc_spi_handle);
    ESP_ERROR_CHECK(ret);
    ESP_LOGI(TAG, "ESC SPI Initialized (Host: %d, MOSI:%d, MISO:%d, CLK:%d, CS:%d)",
             ESC_SPI_HOST, PIN_NUM_MOSI, PIN_NUM_MISO, PIN_NUM_CLK, PIN_NUM_CS);

    // Configure ESC interrupt pin
    gpio_config_t io_conf = {
        .intr_type = GPIO_INTR_NEGEDGE, // Or as specified by ESC/stack (e.g. low level)
        .pin_bit_mask = (1ULL << PIN_NUM_IRQ),
        .mode = GPIO_MODE_INPUT,
        .pull_up_en = GPIO_PULLUP_ENABLE, // Or as required by ESC
    };
    gpio_config(&io_conf);
    // gpio_install_isr_service(0); // ESP_INTR_FLAG_DEFAULT or 0
    // gpio_isr_handler_add(PIN_NUM_IRQ, esc_interrupt_handler, (void*) PIN_NUM_IRQ);
    ESP_LOGI(TAG, "ESC IRQ Pin %d configured. ISR handler conceptual.", PIN_NUM_IRQ);
}

void ethercat_application_task(void *pvParameters) {
    ESP_LOGI(TAG, "EtherCAT Application Task Started.");

    // Initialize application data
    g_app_inputs.input_byte1 = 0xAA;
    g_app_inputs.input_byte2 = 0x55;
    g_app_outputs.output_byte1 = 0x00;
    g_app_outputs.output_byte2 = 0x00;

    // --- Conceptual EtherCAT Slave Stack Initialization ---
    // This is where a real EtherCAT stack (like Beckhoff SSC generated code, or SOEM port)
    // would be initialized. It involves:
    // 1. Initializing the ESC hardware interface (SPI in this case).
    //    The stack would use functions like esc_spi_read/write_reg, esc_spi_read/write_fifo.
    // 2. Setting up ESC registers, PDI, SyncManagers, FMMUs based on ESI configuration.
    // 3. Starting the EtherCAT State Machine handling.
    // 4. Registering application callbacks for process data, state changes, etc.

    // Example (highly conceptual):
    // if (ecat_slave_stack_init(ESC_SPI_HOST, &g_esc_spi_handle, PIN_NUM_IRQ) != ECAT_STACK_OK) {
    //     ESP_LOGE(TAG, "Failed to initialize EtherCAT slave stack!");
    //     vTaskDelete(NULL); return;
    // }
    // ecat_slave_register_pdo_input_buffer(&g_app_inputs, sizeof(g_app_inputs));
    // ecat_slave_register_pdo_output_buffer(&g_app_outputs, sizeof(g_app_outputs));
    // ecat_slave_set_app_hooks(app_read_outputs_from_dpram, app_write_inputs_to_dpram);
    // ecat_slave_stack_start_mainloop(); // This would be a blocking call or start internal tasks

    ESP_LOGW(TAG, "Conceptual EtherCAT stack functions are commented out.");
    ESP_LOGW(TAG, "This example focuses on SPI setup and application data handling concepts.");
    ESP_LOGI(TAG, "A real implementation requires a dedicated EtherCAT slave stack component and ESC hardware.");
    // --- End of Conceptual Stack Initialization ---

    uint8_t counter = 0;
    while (1) {
        // Main application loop for the slave device.
        // The EtherCAT stack runs in the background (often interrupt-driven or in high-priority tasks)
        // and handles all EtherCAT protocol communication with the ESC.

        // 1. Update input process data (g_app_inputs) based on sensor readings, etc.
        g_app_inputs.input_byte1 = counter++;
        // The stack will periodically copy g_app_inputs to the ESC's DPRAM (e.g., triggered by SYNC0).
        // This might be done via app_write_inputs_to_dpram() or directly by the stack.

        // 2. Read output process data (g_app_outputs) that the stack has updated from ESC DPRAM.
        // This might be done via app_read_outputs_from_dpram() or directly by the stack.
        if (g_app_outputs.output_byte1 != 0) { // Example: react to output data
            ESP_LOGI(TAG, "Output Byte 1 changed by Master: 0x%02X", g_app_outputs.output_byte1);
            // TODO: Control actuators based on g_app_outputs
        }

        // Simulate the stack calling our conceptual hooks
        // In a real system, these are called by the stack at appropriate times (e.g., before/after SyncManager updates)
        app_write_inputs_to_dpram(); // Conceptual: stack would take current g_app_inputs
        app_read_outputs_from_dpram(); // Conceptual: stack would have updated g_app_outputs

        ESP_LOGD(TAG, "App Loop: Inputs (0x%02X, 0x%02X), Outputs (0x%02X, 0x%02X)",
                 g_app_inputs.input_byte1, g_app_inputs.input_byte2,
                 g_app_outputs.output_byte1, g_app_outputs.output_byte2);

        vTaskDelay(pdMS_TO_TICKS(100)); // Application logic cycle
    }
}

void app_main(void) {
    // Initialize NVS - good practice, though not directly used in this conceptual example.
    // esp_err_t ret = nvs_flash_init(); ...

    initialize_esc_spi();

    xTaskCreate(ethercat_application_task, "ecat_app_task", 4096 * 2, NULL, 10, NULL);
}

main/CMakeLists.txt

idf_component_register(SRCS "main.c"
                       INCLUDE_DIRS ".")

Build Instructions

1. Save main.c and CMakeLists.txt in the main directory.
2. If using a real EtherCAT stack (e.g., Beckhoff SSC, SOEM port), add it as a component to your project and configure its dependencies (SPI driver, etc.).
3. Configure project settings if needed via ESP-IDF: SDK Configuration editor (menuconfig).
4. Build the project using ESP-IDF: Build your project.

Run/Flash/Observe Steps

1. Flash the compiled firmware to your ESP32 board.
2. Connect the ESP32 (which is connected to the ESC module) to an active EtherCAT master via an Ethernet cable plugged into the ESC module’s Port 0. Ensure the EtherCAT network is properly terminated or looped back.
3. In your EtherCAT Master engineering tool (e.g., TwinCAT):
   • Scan for EtherCAT slaves or add the slave manually using its ESI file.
   • The master should detect the ESP32-hosted ESC.
   • Configure the slave (PDO mapping, DC settings if applicable).
   • Transition the slave through the EtherCAT states (Init -> PreOp -> SafeOp -> Op).
4. Open the ESP-IDF Monitor tool (ESP-IDF: Monitor your device) to observe log messages from the ESP32.
5. On the EtherCAT Master, observe the input data coming from the ESP32 slave and try to write output data to it. You should see the ESP32’s application logic reacting to output data and updating its input data, which is then read by the master.

Tip: An EtherCAT network analysis tool (like Wireshark with EtherCAT dissector, or specialized tools like Beckhoff ET2000) is invaluable for diagnosing communication issues, inspecting telegrams, and checking slave states.

Variant Notes

The ESP32’s role in an EtherCAT slave is primarily as a host controller for an external ESC. The choice of ESP32 variant impacts its ability to perform this role effectively:

General Considerations for all variants:

• SPI Performance: The SPI interface to the ESC is critical. High SPI clock speeds and efficient DMA usage (if supported by the stack and ESC PDI) are important for fast access to the ESC’s DPRAM.
• Interrupt Latency: The ESP32 must respond quickly to interrupts from the ESC (e.g., PDI interrupt, SYNC0/SYNC1 events). Low interrupt latency is key for timely process data updates and state machine handling.
• Memory for Stack and Application: EtherCAT slave stacks, even lean ones designed for ESCs, require some RAM and Flash. The application logic adds to this. Choose a variant with adequate memory. PSRAM can be beneficial for larger applications.
• No Direct EtherCAT via ESP32 Ethernet MAC: It’s crucial to reiterate that the ESP32’s built-in Ethernet MAC cannot be used to directly implement an EtherCAT slave’s physical layer due to the lack of “processing on the fly” hardware. An external ESC is mandatory.

Common Mistakes & Troubleshooting Tips

Exercises

1. Exercise 1: ESC Register Access (Conceptual SPI)
   • Assume you have an ESC connected via SPI. Write a small ESP-IDF function uint16_t read_esc_type_register(spi_device_handle_t spi_handle) that would conceptually read the ESC Type register (Address 0x0000) from the ESC.
   • Describe the SPI transaction(s) needed (command byte for read, address bytes, data bytes). You don’t need to implement the full EtherCAT stack, just the SPI communication to read a 2-byte register. (Hint: Refer to an ESC datasheet like ET1100 for register addresses and SPI access protocol).
2. Exercise 2: ESI File Study – PDO Definition
   • Find an example ESI file online (e.g., from Beckhoff’s ESI download page or an open-source project).
   • Identify a <TxPdo> (transmit PDO, slave inputs) and an <RxPdo> (receive PDO, slave outputs) element.
   • Describe what information is contained within these elements and their <Entry> sub-elements (e.g., Index, SubIndex, BitLen, Name, DataType). How does this define the process data exchanged with the master?
3. Exercise 3: EtherCAT State Machine Logic
   • Your ESP32 application needs to control an LED. The LED should be OFF in Init and PreOp states, blink slowly in SafeOp state, and be controlled by a PDO bit from the master in Op state.
   • Outline the pseudo-code or logic within your ESP32 application (conceptually, how the EtherCAT stack would inform your app of state changes) to implement this behavior based on the current EtherCAT state.
4. Exercise 4: Distributed Clocks (DC) – SYNC0 Event Application
   • Imagine your EtherCAT slave needs to sample an ADC value precisely when the SYNC0 signal from the ESC (triggered by Distributed Clocks) occurs.
   • Describe how the ESP32 application would be structured to:
     1. Be notified of the SYNC0 event (e.g., via an ISR from the ESC’s SYNC0 pin).
     2. Trigger the ADC conversion upon this event.
     3. Make the ADC result available as input process data for the EtherCAT master in the next cycle.

Summary

• EtherCAT is an ultra-high-speed Industrial Ethernet protocol achieving performance via “processing on the fly” and requiring dedicated EtherCAT Slave Controller (ESC) hardware.
• Key EtherCAT concepts include its flexible topology (logically a bus), unique telegram structure with Working Counters, Distributed Clocks for precise synchronization, and a well-defined State Machine (ESM).
• ESI files (XML-based) are crucial for describing slave capabilities to the EtherCAT master.
• An ESP32 cannot be a native EtherCAT slave using its own Ethernet MAC; it must act as a host microcontroller to an external ESC, communicating typically via SPI or a parallel interface (PDI).
• The ESP32 application, with the help of an EtherCAT slave stack library, manages the ESC, handles the ESM, exchanges process data (PDOs) and service data (SDOs), and implements the device-specific logic.
• Successful EtherCAT slave implementation hinges on correct ESC interfacing, accurate ESI files, proper state machine handling, and robust PDO/SDO management.
• While complex, integrating ESP32 with an ESC opens possibilities for creating intelligent, high-performance EtherCAT slave devices.

Further Reading

• EtherCAT Technology Group (ETG): https://www.ethercat.org – The official source for specifications, ESI schema, vendor IDs, technical articles, and news.
• Beckhoff EtherCAT Slave Stack Code (SSC) Tool: https://beckhoff.com/ethercat (Search for SSC Tool) – A tool to generate slave stack C code for various microcontrollers, often used as a starting point.
• ESC Datasheets: (e.g., Beckhoff ET1100/ET1200, Microchip LAN9252/LAN9253) – Essential for understanding ESC registers, PDI, and hardware characteristics.
• SOEM (Simple Open EtherCAT Master): https://github.com/OpenEtherCATsociety/SOEM – An open-source EtherCAT master stack. While for masters, studying its code can provide insights into EtherCAT communication. Some developers have adapted parts for slave experimentation.
• “EtherCAT: The Ethernet Fieldbus” Book by Dr. Jürgen Krey and Florian Wenk (ETG): A comprehensive technical book on EtherCAT.
• ESP-IDF Programming Guide – SPI Master Driver: https://docs.espressif.com/projects/esp-idf/en/v5.4/esp32/api-reference/peripherals/spi_master.html (Select your specific ESP32 target).