Talk to the ESP32 over WiFi

The Serial port is a staple for Arduino programmers. Whether it is debugging, commanding or program output, the simple serial port has you covered.

But what if you want to cut the cable and go wireless? A TCP Server running on the ESP32 is just what you need.

TCP is very similar to a serial port. Both let you send and receive data through a connection between the ESP32 and another computer. There are two main differences:

1. You don’t need any wires for TCP
2. Several computers can connect to the ESP32 at the same time.

This article covers implementing a TCP Server on an ESP32 so you can send text back and forth. With just a few modifications, this will work with the ESP8266 or Arduino MRK WiFi 1010 as well.

What is a TCP Server?

A TCP Server is a part of the software that will run on the ESP32. Its job is to listen for incoming connections at a port on the device’s IP address. When a remote computer, or client, reaches out the TCP Server creates a channel for the ESP32 and the remote client to communicate.

An Arduino program for the ESP32 will use a WiFiServer object to create the server. When a new connection is made, the server will provide a WiFiClient object as the communication channel.

To communicate a client, such as MegunoLink’s TCP Client connection, will connect to the TCP Server using the ESP32’s IP address and the port that the server is listening on. Finding a ESP32’s IP address and port is easier if you setup multicast DNS (mDNS) on the ESP32 to advertise the connection to your network.

Creating a TCP Server

The WiFiServer class in the Arduino library implements a TCP Server for the ESP32. You only need to supply a port for the server to listen on.

The port can be any number between 1 and 65,535, though ports below 1023 are reserved for specific applications. Port 80, for example, is used by web servers. Port 23 is reserved for Telnet, which is an unencrypted text communication protocol. It is a good choice for sending commands to and from your Arduino program.

To create the server and have it listen on port 23, add a WiFiServer variable and call its begin() method:

Accepting Connections

Your ESP32 program must accept connections from remote clients if it wants to communicate with them. The hasClient method of the WiFiServer object will return true whenever a remote computer is trying to connect. To accept the connection, retrieve the client object from the WiFiServer‘s available() method and save it.

The CheckForConnection function looks for a new connection and saves it in the RemoteClient variable. There is only one place to store the connection so we can only communicate with one remote client at a time. If a new connection comes in the program could either close the current connection to accept the new one, or reject the new connection until the current connection is disconnected by the remote. Here, the new connection is rejected.

Normally, you should call CheckForConnections in the loop() method.

Try to avoid long delay(…) as well. You might miss a client trying to connect while waiting for the delay to complete. For example, if this program will spend most of its time in the delay on line 5. It will only check for new connections every second and could miss a connection.

Instead, use our ArduinoTimer library:

Sending Data to Remote Clients

The WiFiClient supports print methods. This means you can could send the message Hello World! to a remote client using code like: RemoteClient.println("Hello World!");

You can also use the WiFiClient as a destination for sending data to MegunoLink visualizers. For example, to send the value from the hall effect sensor inside an ESP32 to a TimePlot visualizer, you can:

Receiving from Remote Clients

Receiving data from connected clients is just as easy with TCP connections as it is for Serial connections. Simply call the available() method on a WiFiClient to see how much data is available. If it isn’t zero, you can call:

• read() to get the next character from the buffer, or
• read(Buffer, BufferSize) to get up to BufferSize characters from the buffer; the number of bytes read is returned.

This example echos any data received from a connected client back to it:

Making the Connection in MegunoLink

Use a TCP Client connection to connect to the TCP Servers on an Arduino. Create and configure the connection with the Connection Manager.

MegunoLink supports connecting to remote devices using a mDNS name, an IP address or a DNS name. mDNS, or multicast DNS, makes it easy to find your devices on a wireless network.

Use the TCP Client connection to connect with the ESP8266/ESP32 program

Recap

TCP Servers let you send and receive data from wireless devices like the ESP32/ESP8266 in much the same way as you’d use a Serial connection on an Arduino Uno. You need to create a WiFiServer object and set the port it will listen for connections on. Check for new clients periodically. Once a client has connected, send it data using the print, write, etc methods. Receive data from a connected client using the available() and read() methods.