UART Serial Interface
The Universal Asynchronous Receiver-Transmitter (UART) is the fundamental communication protocol linking microcontrollers to sensors and modules. It serves as the reliable "nervous system" for Automated Guided Vehicles, enabling precise control over motor drivers, GPS modules, and telemetry systems.
Core Concepts
TX & RX Lines
Communication occurs via two physical wires: Transmit (TX) and Receive (RX). The TX of one device connects directly to the RX of the other, forming a simple point-to-point link.
Baud Rate
The speed of data transfer, measured in bits per second (bps). Both devices must agree on the exact same baud rate (e.g., 9600, 115200) to decode the signal correctly.
Asynchronous
Unlike SPI or I2C, UART does not require a shared clock signal. Synchronization is managed through start and stop bits, simplifying wiring requirements for mobile robots.
Data Packets
Data is transmitted in small packets, typically 8 bits. Each packet is framed by a start bit (low) and one or more stop bits (high) to ensure data integrity.
Voltage Levels
Logic levels vary by implementation. Microcontrollers typically use TTL (0V to 3.3V/5V), while industrial standards like RS-232 use higher voltages (-12V to +12V).
Parity Bits
An optional error-checking mechanism. A parity bit is added to the data packet to ensure the total number of 1s is either even or odd, detecting single-bit transmission errors.
How It Works
UART stands as a cornerstone of robotic architecture due to its simplicity. When a host controller (like a Raspberry Pi or STM32) needs to send a command, it pulls the transmit line low to signal a "Start Bit." This wakes up the receiver.
Following the start bit, the data bits (usually 8) are sent sequentially, least significant bit first. This stream represents the actual command, such as a velocity vector for an AGV wheel or a coordinate set.
Finally, the line is pulled high for the "Stop Bit," indicating the packet is complete. Because the timing is strictly governed by the pre-configured baud rate, slight deviations in clock speed between devices can cause garbage data, highlighting the need for precise configuration.
Real-World Applications
LIDAR Navigation
Many 2D LIDAR sensors used for SLAM (Simultaneous Localization and Mapping) stream distance data points to the main computer via high-speed UART connections.
GPS/GNSS Positioning
Outdoor autonomous mobile robots rely on GPS modules that output NMEA sentences—standard ASCII strings containing latitude and longitude—exclusively over serial ports.
Motor Control Units
Communication between the high-level fleet management computer and low-level motor drivers often happens via UART, sending target RPMs and receiving encoder feedback.
Wireless Telemetry
Radio modules (like XBee or LoRa) used for remote monitoring and emergency stops in warehouses act as "wireless serial cables," transparently bridging UART data over the air.
Frequently Asked Questions
What is the difference between UART, I2C, and SPI?
UART is asynchronous and point-to-point (one-to-one), requiring only two data wires without a clock line. I2C is synchronous and supports multiple devices on a bus with just two wires (SDA/SCL) but is generally slower. SPI is synchronous and the fastest of the three, but requires more wires (clock, data in, data out, and chip select) for each device.
What happens if the baud rates are mismatched?
If the transmitter and receiver operate at different baud rates, the timing for sampling bits will drift. This results in "garbage" characters or framing errors, where the receiver misinterprets data bits as start or stop bits. Communication will fail entirely until rates are matched.
How long can UART cables be in an industrial setting?
Standard TTL UART is designed for short distances, typically between chips on the same PCB or short interconnects (under 30cm). For longer runs across an AGV chassis or factory floor, you must convert the signal to RS-232 (up to 15m) or RS-485 (up to 1200m) to combat noise and voltage drop.
What is Hardware Flow Control (RTS/CTS)?
RTS (Request to Send) and CTS (Clear to Send) are extra wires used to prevent buffer overflows. Before sending data, a device asserts RTS, and waits for the receiver to assert CTS. This is critical in high-reliability AGV systems where processing delays might occur.
Can I connect a 5V sensor to a 3.3V Microcontroller UART?
Generally, no. Connecting a 5V TX line to a 3.3V non-tolerant RX pin can permanently damage the microcontroller. You must use a logic level converter or a voltage divider circuit to safely step down the voltage signals.
What is the "framing error" often seen in diagnostics?
A framing error occurs when the receiver does not detect a valid Stop Bit at the expected time. This is almost always caused by baud rate mismatches, significant electrical noise on the line, or an unstable clock source on one of the devices.
Is UART capable of Full Duplex communication?
Yes. Because there are separate physical wires for Transmission (TX) and Reception (RX), data can flow in both directions simultaneously. This allows an AGV to report telemetry data while simultaneously receiving new movement commands.
Why do we swap TX and RX when connecting two devices?
Communication requires the output of one device to go to the input of the other. Therefore, the Transmit (TX) pin of Device A must connect to the Receive (RX) pin of Device B, and vice versa. Connecting TX to TX will result in no communication.
What is the standard configuration (8N1)?
"8N1" is the most common configuration shorthand. It stands for 8 Data Bits, No Parity, and 1 Stop Bit. While configurable, most modern robotics modules default to this setting, requiring only the baud rate to be adjusted.
How do I debug a UART connection?
The most common tool is a USB-to-TTL serial adapter connected to a PC running terminal software (like PuTTY or CoolTerm). For deeper analysis of timing and logic levels, a Logic Analyzer or Oscilloscope is necessary to visualize the square waves.
Can UART be used in a noisy factory environment?
TTL UART is susceptible to electromagnetic interference (EMI) from motors and welding equipment. In factories, it is standard practice to use shielded cables and convert the signal to RS-485 (differential signaling), which cancels out noise effectively.
Does UART support multiple devices on one line (Daisy Chaining)?
Natively, no. Standard UART is strictly point-to-point. To communicate with multiple devices using a single microcontroller port, you would need to use a multiplexer or convert to a protocol like RS-485 which supports multi-drop bus topologies.