ESP8266-UART-to-UART-Bridge

Transparent UART to UART Bridge using ESP8266.

Before

Microprocessor-1		Microprocessor-2
	UART
TX		RX
RX		TX
GND		GND

Using WiFi

Many similar apps have been released, but they often use AP mode servers and STA mode clients.
In this case, it is necessary to start the AP mode server first.
If the STA mode client is started first, it will fail to connect to the AP.
This project runs both nodes in STA mode.
The IP address of the other side can be found using mDNS.
Therefore, we don't need to worry about the boot order of ESP8266.

Microprocessor-1		ESP8266-1		ROUTER		ESP8266-2		Microprocessor-2
	UART		WiFi		WiFi		UART
TX		RX				TX		RX
RX		TX				RX		TX
GND		GND				GND		GND

Using ESPNOW

This project uses ESPNOW broadcast to find the MAC address of the other side.
There is no need to register the MAC address of the other side.
We don't need to worry about the boot order of ESP8266.

Microprocessor-1		ESP8266-1		ESP8266-2		Microprocessor-2
	UART		ESPNOW		UART
TX		RX		TX		RX
RX		TX		RX		TX
GND		GND		GND		GND

How to use WiFi

Edit Your SSID & Password.
Edit UART boudrate.
Flash sketch to ESP8266-1.
Flash same sketch to ESP8266-2.
Connect Microprocessor-1 to ESP8266-1 via UART.
Connect Microprocessor-2 to ESP8266-2 via UART.
Send data from Microprocessor-1 to ESP8266-1.
Receive data from ESP8266-2 to Microprocessor-2.

You can use ESP8266 as a dedicated WiFi router.
esp_wifi_repeater
https://github.com/martin-ger/esp_wifi_repeater

Microprocessor-1		ESP8266-1		ESP8266-3(esp_wifi_repeater)		ESP8266-2		Microprocessor-2
	UART		WiFi		WiFi		UART
TX		RX				TX		RX
RX		TX				RX		TX
GND		GND				GND		GND

How to use ESPNOW

Edit UART boudrate.
Flash sketch to ESP8266-1.
Flash same sketch to ESP8266-2.
Connect Microprocessor-1 to ESP8266-1 via UART.
Connect Microprocessor-2 to ESP8266-2 via UART.
Send data from Microprocessor-1 to ESP8266-1.
Receive data from ESP8266-2 to Microprocessor-2.

How to disable ESP8266 boot messages

ESP8266 boot rom writes a log to the UART when booting like this:
This message can cause unexpected trouble.

ets Jan  8 2014,rst cause 1, boot mode:(3,7)

load 0x40100000, len 24236, room 16
tail 12
chksum 0xb7
ho 0 tail 12 room 4
load 0x3ffe8000, len 3008, room 12
tail 4
chksum 0x2c
load 0x3ffe8bc0, len 4816, room 4
tail 12
chksum 0x46
csum 0x46

Swap UART

UART can be swapped by adding the following one line:

void setup() {
	Serial.swap();   

TX / RX changes to the following when UART is swapped.

GPIO15(TXD)   --- RX
GPIO13(RXD)   --- TX

It doesn't help because the boot messages appear before the main module starts.

Add a circuit

If the following circuit is added, All messages does not reach RX port of the other side.
I used 1N5819 Schottky Barrier Plastic Rectifier.

To start transmission, set the GPIO polarity to OUTPUT and the level to LOW.
This method works pretty reliably.

void setup() {
	pinMode(GPIO, OUTPUT);
	digitalWrite(GPIO, LOW);
	Serial.begin(115200);

NOTE:GPIO15 is pulled down and cannot be used for this.