UART Echo

UART (serial) is the classic way for devices to talk to each other. Here the ESP32 echoes back every character it receives.

Parts

A USB-to-serial converter module (3.3 V logic)

Circuit

Breadboard diagram: a USB serial module wired TX to RX, RX to TX, GND to GND and 3V3 to 3V3

USB serial module	ESP32-DevKitC
TX	RX
RX	TX
GND	GND
3V3	3V3
5V	not connected

TX and RX are crossed: one side’s transmitter goes to the other side’s receiver.

Note: RX/TX on the DevKitC are GPIO3/GPIO1, i.e. UART0, which the board’s on-board USB bridge also uses. Both connections share these pins.

Program

require 'uart'

uart = UART.new(unit: 'ESP32_UART0', txd_pin: 1, rxd_pin: 3)
loop do
  str = uart.read
  unless str.nil?
    uart.write(str)
  end
  sleep_ms(10)
end

The program reads whatever arrives and writes it straight back.

Run

Plug the converter’s USB into your PC. It shows up as a second serial port. Open it with a terminal program, for example:

$ screen /dev/ttyUSB0

(The device name differs by OS, e.g. /dev/tty.usbserial-* on macOS.)

Then save the program as uart.rb, put it in /home on the device (see File Transfer), and run it from the shell:

$> ./uart.rb

Type in the screen window: the characters you type are echoed back.