Ethernet UDP receiving problem

Hello! The packets are almost always recieved, but sometimes after receiving a packet lights LINK, 100M, FULLO and COLL will just turn off for a second and then turn on back again and then the LINK light would just start to rapidly blink and the arduino just becomes unresponsive and won't receive packets. What causes that?


#include <Ethernet.h>
#include <EthernetUdp.h>
#include <SPI.h>

byte mac[] = {0x00, 0xAA, 0xBB, 0xCC, 0xDE, 0x02};
unsigned int localPort = 8888;
char packetBuffer[UDP_TX_PACKET_MAX_SIZE];
char replyBuffer[UDP_TX_PACKET_MAX_SIZE];
EthernetUDP Udp;

void setup() {
// put your setup code here, to run once:
// Serijski port
Serial.println("Serijski port odprt!");

digitalWrite(9, HIGH);
pinMode(9, OUTPUT);

// Nastavitve ethernet povezave
boolean receiving = false;
Serial.println("UDP zagnan!");

void loop() {
// put your main code here, to run repeatedly:
// UDP prejemanje/oddajanje
int packetSize = Udp.parsePacket();
if(packetSize) {, UDP_TX_PACKET_MAX_SIZE);
if(strcmp(packetBuffer, "mode") == 0) {
if(digitalRead(9) == LOW) {
digitalWrite(9, HIGH);
else {
digitalWrite(9, LOW);

Is that an original Arduino Ethernet Shield or some clone? If the later: post schematics. From the description I would guess for a hardware problem.

Ye, it's a clone of Ethernet shield from aliexpress...

With several of these clones there are known problems. Some have wrong resistor values on board. If you don't have schematics of your board (in my experience almost in every case on Aliexpress) try to identify your type of board and search for it on Google. You might find a hint there.