01 Overview

UART Echo is the starting point for serial debugging: read a byte, write it back. It doesn't implement any business protocol, only proves pins, baud, and wiring.

Use it when bringing up a second UART, connecting to meters, PLCs, or wireless modules. Don't rush to write protocol stacks before confirming the physical link.

Skip it when the protocol is already stable; add Modbus or custom frame parsing on top of echo, not permanent echo.

Hardware-wise, TTL to TTL is a direct cross connection; RS485 requires a transceiver and termination resistor. Common ground is mandatory.

Software-wise, ESP-IDF's UART driver provides event loops and buffer management for async RX/TX.

This is a public ESP-IDF example for verifying basic serial functionality, not product-specific logic.

02 Hardware

• MCU: ESP32 family, any UART-capable pins.
• Levels: TTL 3.3V, cross TX/RX. Never raw RS232 voltages.
• RS485: Needs a transceiver (e.g., MAX485), A/B differential lines, 120Ω termination, DE/RE control.
• Ground: Must share ground with the peer, or you get garbage.
• Power: If powered by peer, check level matching and isolation.
• Wiring: See schematic for pin assignments.
• Layout: For long lines, consider shielding and termination.

03 Software flow

1. Configure UART port, baud, data bits, stop bits, parity, and buffer sizes.

2. Install driver and set pin mapping (TX, RX, optional RTS/CTS).

3. Create a read task using uart_read_bytes.

4. Write back the read data using uart_write_bytes.

5. Handle error events (e.g., buffer overflow, disconnection).

6. In products, replace with frame parsing, not permanent echo.

04 Core points

• Garbage is usually ground or levels, then firmware.
• RS485 requires DE/RE direction control to avoid contention.
• Custom work: RS485 hardware, protocol, isolation.
• Don't copy directly to other chips; pins and drivers differ.
• Adjust buffers based on baud and line length.