UART (Universal Asynchronous Receiver/Transmitter) is technically neither; it is the logic protocol itself, but it almost always operates at TTL levels (\(0V\)-\(3.3V/5V\)) inside microcontrollers. It produces raw data, which can then be transmitted via TTL or converted to RS-232 (\(+12V\) to \(-12V\)) for long-distance industrial communication. [1, 2, 3, 4, 5]
Key Differences:
- UART (TTL): Used on microcontrollers (Arduino, STM32) and sensors. Idle state is HIGH (\(3.3V/5V\)), and active is LOW (\(0V\)).
- RS-232: Used on older PCs and industrial equipment. Uses high bipolar voltages (e.g., \(\pm 12V\)) for long distances and noise immunity.
- Compatibility: You must use a MAX232 level shifter to connect a TTL UART to an RS-232 device.
- TTL UART (0–Vcc, e.g., 3.3V) is for short-distance communication on a single board.
- RS-232 (\(\pm 3V\) to \(\pm 15V\)) is for long-distance, industrial, or legacy communication. [1, 2, 3, 4, 5, 6]
Important: Never connect a TTL UART directly to an RS-232 port, as the higher voltages can damage your microcontroller
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