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Thread: Ethernet Teensy 4.1 connection problem - NativeEthernet bug?

  1. #1
    Junior Member
    Join Date
    Nov 2020

    Ethernet Teensy 4.1 connection problem - NativeEthernet bug?

    In a project we are using Teensy 4.1 as a host to control some hardware via network. Sometimes we loose connection. To reproduce the behavior, I modified the AdvancedChatserver example to behave like our software. It disconnects any client except the authorized one if there are more than six clients, instead of checking for disconnected clients as the original example does. A C# testapp creates multiple clients that just connect to the Teensy by socket.connect() and receive the "we have a new client" response. Sometimes Teensy does no longer respond and clients don't get the message. Then Teensy needs to reboot before any new client can connect. Has anyone else seen something similar? Is it a bug in the NativeEthernet library or a hardware problem? Below is the code of the modified AdvancedChatserver example.

     Advanced Chat Server
     A more advanced server that distributes any incoming messages
     to all connected clients but the client the message comes from.
     To use, telnet to your device's IP address and type.
     You can see the client's input in the serial monitor as well.
     Using an Arduino Wiznet Ethernet shield.
     * Ethernet shield attached to pins 10, 11, 12, 13
     created 18 Dec 2009
     by David A. Mellis
     modified 9 Apr 2012
     by Tom Igoe
     redesigned to make use of operator== 25 Nov 2013
     by Norbert Truchsess
    //#include <SPI.h>
    #include <NativeEthernet.h>
    // Enter a MAC address and IP address for your controller below.
    // The IP address will be dependent on your local network.
    // gateway and subnet are optional:
    byte mac[] = {
      0x05, 0x39, 0x22, 0x11, 0x04, 0x23
    IPAddress ip(192, 168, 5, 55);
    IPAddress myDns(192, 168, 99, 254);
    IPAddress gateway(192, 168, 99, 254);
    IPAddress subnet(255, 255, 255, 0);
    // telnet defaults to port 23
    EthernetServer server(23);
    int ClientCount;
    int lastClient;
    EthernetClient clients[8];
    void setup() {
      // You can use Ethernet.init(pin) to configure the CS pin
      //Ethernet.init(10);  // Most Arduino shields
      //Ethernet.init(5);   // MKR ETH shield
      //Ethernet.init(0);   // Teensy 2.0
      //Ethernet.init(20);  // Teensy++ 2.0
      //Ethernet.init(15);  // ESP8266 with Adafruit Featherwing Ethernet
      //Ethernet.init(33);  // ESP32 with Adafruit Featherwing Ethernet
      // initialize the Ethernet device
      Ethernet.begin(mac, ip, myDns, gateway, subnet);
      // Open serial communications and wait for port to open:
      while (!Serial) {
        ; // wait for serial port to connect. Needed for native USB port only
      // Check for Ethernet hardware present
      if (Ethernet.hardwareStatus() == EthernetNoHardware) {
        Serial.println("Ethernet shield was not found.  Sorry, can't run without hardware. :(");
        while (true) {
          delay(1); // do nothing, no point running without Ethernet hardware
      if (Ethernet.linkStatus() == LinkOFF) {
        Serial.println("Ethernet cable is not connected.");
      // start listening for clients
      Serial.print("Chat server address:");
    void loop() {
      // check for any new client connecting, and say hello (before any incoming data)
      lastClient = -1;
      EthernetClient newClient = server.accept();
      if (newClient) {
        for (byte i=0; i < 8; i++) {
          if (!clients[i]) {
            Serial.print("We have a new client #");
            newClient.print("Hello, client number: ");
            // Once we "accept", the client is no longer tracked by EthernetServer
            // so we must store it into our list of clients
            clients[i] = newClient;
            lastClient = i;
      // check for incoming data from all clients
      for (byte i=0; i < 8; i++) {
        if (clients[i] && clients[i].available() > 0) {
          // read bytes from a client
          byte buffer[80];
          int count = clients[i].read(buffer, 80);
          // write the bytes to all other connected clients
          for (byte j=0; j < 8; j++) {
            if (j != i && clients[j].connected()) {
              clients[j].write(buffer, count);
      // stop any clients which disconnect
      //for (byte i=0; i < 8; i++) {
      //  if (clients[i] && !clients[i].connected()) {
      //    Serial.print("disconnect client #");
      //    Serial.println(i);
      //    clients[i].stop();
      //  }
      ClientCount = 0;
        for (byte i=0; i < 8; i++) {
        if (clients[i] && clients[i].connected()) {
        if(ClientCount > 6){
          for (byte i=0; i < 8; i++){
            if(clients[i] && i != lastClient){
              Serial.print("disconnect client #");
    Last edited by defragster; 03-08-2023 at 08:37 AM. Reason: edit to use # CODE markers

  2. #2
    Senior Member
    Join Date
    Mar 2017
    Oakland, CA, USA
    Have a look at QNEthernet. Keep in mind some extra conveniences with the library, for example, you donít need to specify a MAC address.

  3. #3
    1+ for using QNEthernet. It uses more RAM, but it's more stable and has more options than NativeEthernet. I had better luck with QNEthernet in my projects. Also, it's more actively maintained.

  4. #4
    Junior Member
    Join Date
    Nov 2020
    Quote Originally Posted by shawn View Post
    Have a look at QNEthernet. Keep in mind some extra conveniences with the library, for example, you donít need to specify a MAC address.
    Thanks, first tests look promising. So far no connection problems with the modified AdvancedChatServer example. Hopefully it also works in our project.

  5. #5
    Senior Member
    Join Date
    Mar 2017
    Oakland, CA, USA
    The latest QNEthernet push further reduces RAM1 usage by about 27KiB with the current configuration by moving lwIP's memory pools to RAM2, along with some other hundreds-of-bytes changes.

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