In the world of embedded development and industrial automation, terminology is often used loosely. It is very common to hear “UART” and “RS-232” used as synonyms in casual technical discussions.
For example, a requirement might call for an “RS-232 interface.” However, after clarifying the technical details, the actual need might be a simple logic-level connection for debugging or communication between microcontrollers on the same circuit board.
To a Firmware or Hardware Engineer, these terms represent two different layers of communication: the protocol (the logic) and the physical interface (the voltage, pinout, and cabling). Distinguishing between them helps ensure that hardware specifications match the actual system requirements and leads to smoother integration.
Let’s look at the difference between the logic of the conversation (UART) and the physical medium it travels through (RS-232).
UART
UART stands for Universal Asynchronous Receiver-Transmitter.
Strictly speaking, a UART is not a communication standard like Wi-Fi or Ethernet. It is usually a hardware peripheral block inside a microcontroller (MCU). Its job is to format data. It takes parallel bytes from the CPU and serializes them into a stream of bits, framing them with start bits, stop bits, and optional parity to be transmitted via a pin.
When a UART transmits data directly from the microcontroller’s pins, the signal exists at Logic Levels. Historically, these were known as TTL (Transistor-Transistor Logic) levels, which typically operated at 5V. Modern microcontrollers, however, are predominantly operating at lower voltages like 3.3V or 1.8V.
Because of these voltage differences, compatibility is a key consideration. Connecting a legacy 5V signal directly to a modern 3.3V microcontroller can damage internal circuitry. It is worth verifying if specific pins can handle 5V signals. If not, you’ll have to create a logic level shifter to bridge the two voltage domains safely.
In practice, UART is the workhorse of on-board communication. It is used to connect the main microcontroller to peripherals located on the same PCB: such as Wi-Fi modules, GPS receivers, modems, etc.
It is also the primary interface for hardware bring-up and debugging, allowing developers to view system logs and interact with the firmware via a simple serial terminal.
For this purpose, a USB‑to‑UART adapter becomes an essential tool. These adapters allow a developer to connect a PC directly to the microcontroller’s UART pins using a standard USB port. On the PC side, the adapter enumerates as a virtual COM port, which can be accessed using common terminal programs such as RealTerm. On the hardware side, the adapter outputs logic‑level UART signals (typically 3.3 V or 5 V), making it safe to connect directly to a PCB’s debug header. One of the examples of such USB-UART adapters is presented below.
USART (Universal Synchronous/Asynchronous Receiver-Transmitter)
If you look at datasheets from manufacturers like STMicroelectronics or Microchip, you will often see the peripheral labeled as USART. The “S” stands for Synchronous, meaning the hardware can support a clock line (CLK). Therefore, such USART peripherals can be configured to support common Synchronous protocols, such as SPI (Serial Peripheral Interface), and others. However, this peripheral can also be configured in Asynchronous mode, where it behaves exactly like a standard UART.
RS-232
RS-232 (Recommended Standard 232) defines the physical interface. It describes how the data is electrically transmitted to work reliably over cables in the “outside world,” away from the protection of the PCB.
RS-232 transmits the same data packet as the UART, but with two major physical differences designed to improve noise immunity over longer cables.
- Voltage Levels – RS-232 uses bipolar signals. A valid signal typically ranges from ±3V to ±15V.
- Signal Inversion – This is a common point of confusion. The logic is inverted compared to UART:
- Logic ‘1’ (Mark) is represented by a Negative voltage (e.g., -12V).
- Logic ‘0’ (Space) is represented by a Positive voltage (e.g., +12V).
- Therefore, while the UART signal idles High (Vcc), the RS-232 signal idles Low (Negative Voltage).
Despite its age, RS-232 remains ubiquitous in industrial and commercial environments due to its robustness and simplicity. It can be found in industrial automation for programming PLCs (Programmable Logic Controllers), other industrial and scientific equipment. Additionally, RS-232 serves as the standard “Console” interface on some networking equipment, Point of Sale (POS) peripherals like receipt printers and barcode scanners.
RS-232 ports used to be built into most PCs and laptops, but not anymore. Therefore, to interface modern computers with legacy equipment, a USB‑to‑RS‑232 adapter is typically used. These adapters bridge the gap between modern USB port and RS‑232 devices. On the PC side, the adapter appears as a virtual COM port to allow unified communication with RS-232 devices. Most USB-to-RS-232 adapters terminate in a 9pin DE9 connector (commonly referred to as DB9).
It is worth noting that while the modern standard is the 9-pin DE9 (often called DB9) connector, the original standard defined a 25-pin connector (DB25), which you can still find on some very legacy devices.
Connecting UART to RS-232
Since a microcontroller cannot natively generate negative voltages or handle 12V signals, we cannot wire a standard RS-232 DE9 connector directly to the MCU. To implement an RS-232 interface, we use a Transceiver IC (such as the industry-standard MAX232 or similar).
- MCU Side – The UART peripheral is configured to use TX (Transmit) and RX (Receive) pins at MCU logic levels.
- Transceiver – The transceiver IC (MAX232 or similar) sits between the MCU and the RS-232 connector. It generates the required high voltages and handles the signal conversion.
- Connector – The transceiver IC outputs standard RS-232 signals to the port (DE9 connector).
Such adapters are available as standalone boards that you can use for quick prototyping and experimentation. One of the options is presented below.
Flow Control and Pinout
Many common integrations often only require a minimal connection consisting of TX, RX, and Ground. In these cases, software flow control (referred as XON/XOFF) is sometimes used to prevent sending too much data too fast and overflowing the receiver’s buffer, or the data rate is just low enough and the receiver can handle it fast enough that buffers do not overflow.
However, Hardware Flow Control is not just a relic of the past. It is widely used in modern equipment to ensure data integrity. In this scenario, the firmware engineer configures two additional pins: RTS (Request to Send) and CTS (Clear to Send). These signals are also routed through the transceiver to help manage the data flow. These signals are standard on both DE9 and DB25 RS-232 connectors.
Cabling Configuration: DTE vs. DCE
Another common pitfall involves cabling. The RS-232 standard historically distinguishes between Data Terminal Equipment (DTE), such as computers, and Data Communication Equipment (DCE), such as modems. While a straight-through cable connects a DTE to a DCE, connecting two DTE devices directly (e.g., a PC to an embedded system) requires a Null Modem cable or adapter to cross the transmit and receive lines correctly.
On a related note, when connecting two UART devices, it is crucial to remember the crossover rule: the TX pin of the transmitting device must connect to the RX pin of the receiving device, and vice versa.
Summary
Here is a quick breakdown of the key distinction:
- UART is the protocol logic inside the chip. It is typically used for on-board communication or direct debugging.
- RS-232 is the physical interface, most easily recognized by the standard DE9/DB9 connector. It is commonly employed to connect devices over external cables, using a transceiver to handle higher voltages and signal inversion.
Understanding where the logic ends and the physical layer begins helps design more robust systems and avoid compatibility issues.
DMC’s embedded team has extensive experience bridging the gap between low-level firmware and complex hardware integration. If you need assistance with your next embedded development project, contact us today.