How to make MQTT and ArduinoModbus libraries working on the same Ethernet port?

I have a SAMD51P20 based Arduino PLC with WIZnet W5500 based Ethernet shield. I'm using these libraries:

#include <Ethernet.h> #include <MQTT.h> #include <ArduinoModbus.h> 

MQTT and Modbus-TCP clients run fine individually, but running them at the same time creates problems with both of them.

Is there a software solution to this problem, e.g. using separate sockets for MQTT and Modbus-TCP. WIZnet W5500 has 8 sockets available, but I don't see an API to select a socket for MQTT and Modbus-TCP client.

If not software, is there a hardware solution?

Here is the code snippet:

#include <fmt.h> #include <P1AM.h> #include <Ethernet.h> #include <MQTT.h> #include <ArduinoModbus.h> using namespace std; #define HOST_NAME "WIZnet" uint8_t macPLC[] = {0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED}; uint8_t ipPLC[] = {192, 168, 0, 41}; uint8_t ipModbus[] = {192, 168, 0, 43}; char ipMQTT[] = "192.168.0.42"; int unitID = 98; EthernetClient ethernet; MQTTClient mqtt; ModbusTCPClient modbus(ethernet); int n = 0; void messageReceived(MQTTClient *client, char topic[], char bytes[], int length) { if (length == 8) { int32_t commandID; float commandArg; memcpy(&commandID, &bytes[0], sizeof(commandID)); memcpy(&commandArg, &bytes[4], sizeof(commandArg)); Serial.println(fmt::format("{} {}", commandID, commandArg).c_str()); } else Serial.println("wrong length!!!"); } void servoSetup() { modbus.coilWrite(unitID, 0x00C, 1); } void setup(){ // the setup routine runs once: P1.configWD(10000, 1); P1.startWD(); Serial.begin(115200); //initialize serial communication at 115200 bits per second while (!P1.init()) delay(1); //Wait for Modules to Sign on Ethernet.begin(macPLC, ipPLC); mqtt.begin(ipMQTT, ethernet); modbus.setTimeout(500); if (!modbus.begin(ipModbus, 502)) Serial.println("Failed to start Modbus TCP client!"); else Serial.println("Connected to Modbus TCP client."); P1.petWD(); while (!mqtt.connect(ipMQTT)) { Serial.print("."); delay(1000); } Serial.println("\nConnected to MQTT broker."); mqtt.subscribe("plc", 2); mqtt.onMessageAdvanced(messageReceived); servoSetup(); delay(2000); } void loop() { P1.petWD(); Serial.println(mqtt.loop()); Serial.println(n++); int addr = 0x2407; if (!modbus.holdingRegisterWrite(unitID, addr, n)) Serial.println(fmt::format("Modbus write failed: {}", modbus.lastError()).c_str()); delay(1000); if (auto result = modbus.holdingRegisterRead(unitID, addr); result < 0) Serial.println(fmt::format("Modbus read failed: {}", modbus.lastError()).c_str()); else Serial.println(fmt::format("holdingRegisterRead: {}", result).c_str()); delay(1000); } 

In this case, Modbus-TCP writes and reads have no effect, mqtt.loop() returns 0 and messageReceived() callback is not called. If I comment out MQTT or Modbus-TCP client, the code behaves as expected.