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

Question

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.

• hcheung: What kind of problem? The 3-line of code doesn't tell us anything. You need to elaborate more on how you create the client for each connection?
• Paul Jurczak: @hcheung I posted the full test code.
• hcheung: ModbusTCPClient modbus(ethernet); and mqtt.begin(ipMQTT, ethernet);, you are sharing the same Ethernet client for two services. Create a separate client instance for each service.
• Paul Jurczak: @hcheung Thank you for looking into it.

Answer 1

There is no API to set the count of sockets used in the Ethernet library.

The MAX_SOCK_NUM is set in Ethernet.h based on available RAM.

#if defined(RAMEND) && defined(RAMSTART) && ((RAMEND - RAMSTART) <= 2048)
#define MAX_SOCK_NUM 4
#else
#define MAX_SOCK_NUM 8
#endif

So on SAMD51 you have 8 sockets available. Your problem with MQTT and Modbus TCP is elsewhere.

btw. the W5100 can handle only 4 sockets, but that is constrained at runtime.

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EDIT: In your sketch you use the same EthernetClient for both libraries. Use two separate EthernetClient objects.

EthernetClient ethernet;
EthernetClient modbusClient;
MQTTClient mqtt;
ModbusTCPClient modbus(modbusClient);