esp-idf/examples/peripherals/uart/uart_echo_rs485

Supported Targets	ESP32	ESP32-C2	ESP32-C3	ESP32-C5	ESP32-C6	ESP32-H2	ESP32-P4	ESP32-S2	ESP32-S3

UART RS485 Echo Example

(See the README.md file in the upper level 'examples' directory for more information about examples.)

This is an example which echoes any data it receives on UART port back to the sender in the RS485 network. It uses ESP-IDF UART software driver in RS485 half duplex transmission mode and requires external connection of bus drivers. The approach demonstrated in this example can be used in user application to transmit/receive data in RS485 networks.

How to use example

Hardware Required

PC + USB Serial adapter connected to USB port + RS485 line drivers + Espressif development board. The MAX483 line driver is used for example below but other similar chips can be used as well.

RS485 example connection circuit schematic:

         VCC ---------------+                               +--------------- VCC
                            |                               |
                    +-------x-------+               +-------x-------+
         RXD <------| RO            |               |             RO|-----> RXD
                    |              B|---------------|B              |
         TXD ------>| DI  MAX483    |    \  /       |    MAX483   DI|<----- TXD
ESP32 BOARD         |               |   RS-485 side |               |  SERIAL ADAPTER SIDE
         RTS --+--->| DE            |    /  \       |             DE|---+
               |    |              A|---------------|A              |   |
               +----| /RE           |               |            /RE|---+-- RTS
                    +-------x-------+               +-------x-------+
                            |                               |
                           ---                             ---

Connect an external RS485 serial interface to an ESP32 board

Connect a USB-to-RS485 adapter to a computer, then connect the adapter's A/B output lines with the corresponding A/B output lines of the RS485 line driver connected to the ESP32 chip (see figure above).

  ------------------------------------------------------------------------------------------------------------------------------
  |  UART Interface       | #define            | Default pin for ESP32 | Default pins for others   | External RS485 Driver Pin |
  | ----------------------|--------------------|-----------------------|---------------------------|---------------------------|
  | Transmit Data (TxD)   | CONFIG_MB_UART_TXD | GPIO23                | GPIO9                     | DI                        |
  | Receive Data (RxD)    | CONFIG_MB_UART_RXD | GPIO22                | GPIO8                     | RO                        |
  | Request To Send (RTS) | CONFIG_MB_UART_RTS | GPIO18                | GPIO10                    | ~RE/DE                    |
  | Ground                | n/a                | GND                   | GND                       | GND                       |
  ------------------------------------------------------------------------------------------------------------------------------

Note: Each target chip has different GPIO pins available for UART connection. Please refer to UART documentation for selected target for more information.

Configure the project

idf.py menuconfig

Build and Flash

Build the project and flash it to the board, then run monitor tool to view serial output:

idf.py -p PORT flash monitor

(To exit the serial monitor, type Ctrl-].)

See the Getting Started Guide for full steps to configure and use ESP-IDF to build projects.

Setup external terminal software

Refer to the example and set up a serial terminal program to the same settings as of UART in ESP32-WROVER-KIT board. Open the external serial interface in the terminal. By default if no any symbols are received, the application sends character . to check transmission side. When typing message and push send button in the terminal you should see the message RS485 Received: [ your message ], where "your message" is the message you sent from terminal. Verify if echo indeed comes from your board by disconnecting either TxD or RxD pin. Once done there should be no any . displayed.

Example Output

Example output of the application:

I (655020) RS485_ECHO_APP: Received 12 bytes:
[ 0x79 0x6F 0x75 0x72 0x20 0x6D 0x65 0x73 0x73 0x61 0x67 0x65 ]

The received message is showed in hexadecimal form in the brackets.

Troubleshooting

When example software does not show the . symbol, the issue is most likely related to connection errors of the external RS485 interface. Check the RS485 interface connection with the environment according to schematic above and restart the application. Then start terminal software and open the appropriate serial port.