Layer: L7 Application · Build flags: PROTOCORE_ENABLE_OPCUA

What this example teaches

OPC UA is the dominant industrial interoperability protocol. This opens an OPC UA Binary endpoint on TCP/4840 implementing the type codec, UA-TCP (UACP) framing, the Hello/Acknowledge handshake, the SecureChannel (OpenSecureChannel, SecurityPolicy None), the Session (CreateSession + ActivateSession), and the Read / Write / Browse services - so a real client (UaExpert, open62541, Python asyncua) completes the handshake, opens a secure channel, activates a session, browses the Objects folder, and reads/writes node values. Like Modbus (ModbusTcp), it is just another protocol on its own port, sharing the HTTP server's pool and event loop.

Three resolvers map a tiny address space onto live values. Read returns a value for a node id (false -> BadNodeIdUnknown):

static bool protocore_opcua_read(uint16_t ns, uint32_t id, uint32_t attribute, OpcUaVariant *out) {
    if (ns != 1 || attribute != OPCUA_ATTR_VALUE) return false;
    switch (id) {
    case 1:  out->type = OpcUaVariantType::OPCUA_VAR_UINT32; out->u32 = millis() / 1000;    return true; // Uptime
    case 2:  out->type = OpcUaVariantType::OPCUA_VAR_UINT32; out->u32 = ESP.getFreeHeap();  return true; // FreeHeap
    case 10: out->type = OpcUaVariantType::OPCUA_VAR_UINT32; out->u32 = setpoint;           return true; // writable
    default: return false;
    }
}

Write returns an OPC UA StatusCode - 0 (Good) on success, or a Bad code the client surfaces:

static uint32_t protocore_opcua_write(uint16_t ns, uint32_t id, uint32_t attribute, const OpcUaVariant *value) {
    if (ns == 1 && id == 10 && attribute == OPCUA_ATTR_VALUE && value->type == OpcUaVariantType::OPCUA_VAR_UINT32) {
        setpoint = value->u32; return 0; // Good
    }
    return (ns == 1 && id == 10) ? OPCUA_STATUS_BAD_NOT_WRITABLE : OPCUA_STATUS_BAD_NODE_ID_UNKNOWN;
}

Browse fills the references under the standard Objects folder (ns=0;i=85) so the variables show up in a client's address-space tree.

Register the handlers, then listen before begin():

protocore_opcua_set_read_handler(protocore_opcua_read);
protocore_opcua_set_write_handler(protocore_opcua_write);
protocore_opcua_set_browse_handler(protocore_opcua_browse);
server.listen(4840, ProtoConn::PROTO_OPCUA); // before begin() - it activates the listeners
server.begin(80);

Build and run

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPROTOCORE_ENABLE_OPCUA=1" \
  --lib="." examples/L7-Application/OpcUa/OpcUa.ino

Connect a client to opc.tcp://<ip>:4840 (UaExpert, or python -m asyncua.tools.uals -u opc.tcp://<ip>:4840) and browse/read the Objects folder.

Annotated source

The complete sketch (OpcUa.ino), reproduced verbatim with added explanatory comments:

// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
#define PROTOCORE_ENABLE_OPCUA 1
 
#include "protocore.h"
#include "network_drivers/physical/physical/physical.h"
#include "services/opcua/opcua.h"
 
static const char *SSID = "YOUR_SSID";
static const char *PASSWORD = "YOUR_PASSWORD";
 
PC server;
 
static uint32_t setpoint = 100; // a writable variable, exposed at ns=1;i=10
 
// Read resolver: map a tiny address space (namespace 1) onto live values. A client
// Reads node ns=1;i=1 (uptime), i=2 (free heap), i=3 (a Double) or i=10 (the writable
// setpoint). Return false for anything else and the server answers BadNodeIdUnknown.
static bool protocore_opcua_read(uint16_t ns, uint32_t id, uint32_t attribute, OpcUaVariant *out)
{
    if (ns != 1 || attribute != OPCUA_ATTR_VALUE)
        return false;
    switch (id)
    {
    case 1:
        out->type = OpcUaVariantType::OPCUA_VAR_UINT32;
        out->u32 = millis() / 1000;
        return true;
    case 2:
        out->type = OpcUaVariantType::OPCUA_VAR_UINT32;
        out->u32 = ESP.getFreeHeap();
        return true;
    case 3:
        out->type = OpcUaVariantType::OPCUA_VAR_DOUBLE;
        out->f64 = 23.5;
        return true;
    case 10:
        out->type = OpcUaVariantType::OPCUA_VAR_UINT32;
        out->u32 = setpoint;
        return true;
    default:
        return false;
    }
}
 
// Write resolver: only ns=1;i=10 (the setpoint) is writable. Returns a StatusCode -
// Good (0) on success, or a Bad code the client surfaces as the write result.
static uint32_t protocore_opcua_write(uint16_t ns, uint32_t id, uint32_t attribute, const OpcUaVariant *value)
{
    if (ns == 1 && id == 10 && attribute == OPCUA_ATTR_VALUE && value->type == OpcUaVariantType::OPCUA_VAR_UINT32)
    {
        setpoint = value->u32;
        return 0; // Good
    }
    return (ns == 1 && id == 10) ? OPCUA_STATUS_BAD_NOT_WRITABLE : OPCUA_STATUS_BAD_NODE_ID_UNKNOWN;
}
 
// Browse resolver: the standard Objects folder (ns=0;i=85) organizes our three
// variables (ns=1;i=1..3). Browsing anything else is BadNodeIdUnknown.
static int32_t protocore_opcua_browse(uint16_t ns, uint32_t id, OpcUaReference *out, uint32_t max)
{
    if (ns != 0 || id != 85) // i=85 == Objects folder
        return -1;
    static const char *names[3] = {"Uptime", "FreeHeap", "Temperature"};
    int32_t n = 0;
    for (uint32_t i = 0; i < 3 && (uint32_t)n < max; i++, n++)
    {
        out[n].ref_type_id = OPCUA_REFTYPE_ORGANIZES;
        out[n].is_forward = true;
        out[n].target_ns = 1;
        out[n].target_id = i + 1;
        out[n].browse_name_ns = 1;
        out[n].browse_name = names[i];
        out[n].display_name = names[i];
        out[n].node_class = OPCUA_NODECLASS_VARIABLE;
        out[n].type_def_id = OPCUA_TYPEDEF_BASE_DATA_VARIABLE;
    }
    return n;
}
 
void setup()
{
    Serial.begin(115200);
    Physical.wifi->init(SSID, PASSWORD);
    while (!Physical.wifi->ready())
        delay(250);
    Serial.print("IP: ");
    uint32_t ip = Physical.link->egress_ip(); // library egress IP (network byte order), no Arduino WiFi
    Serial.printf("IP: %u.%u.%u.%u\n", (unsigned)(ip & 0xFF), (unsigned)((ip >> 8) & 0xFF),
                  (unsigned)((ip >> 16) & 0xFF), (unsigned)((ip >> 24) & 0xFF));
 
    server.on("/", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) { server.send(id, 200, "text/plain", "OPC UA on :4840"); });
    protocore_opcua_set_read_handler(protocore_opcua_read);     // serve Reads for ns=1;i=1..3,10
    protocore_opcua_set_write_handler(protocore_opcua_write);   // accept Writes to ns=1;i=10 (the setpoint)
    protocore_opcua_set_browse_handler(protocore_opcua_browse); // list those under the Objects folder
    server.listen(4840, ProtoConn::PROTO_OPCUA);       // OPC UA Binary endpoint - before begin() (it activates listeners)
    server.begin(80);
    Serial.println("OPC UA endpoint: opc.tcp://<ip>:4840 (handshake + SecureChannel + Session + Read/Write + Browse)");
}
 
void loop()
{
    server.handle();
}