UART and RS-232, RS-422, RS-485 links and differences
Serial communication is one of the most fundamental methods that electronic engineers encounter. Among these, RS-232 is considered the simplest. Many beginners often get confused about the differences between UART and standards like RS-232, RS-422, and RS-485. This article aims to clarify these concepts and help you understand their relationships.
If we compare serial communication to traffic, then UART can be thought of as a station. Each frame of data is like a car traveling on the road, following specific rules. In a city, speed limits might be 30 or 40, while on the highway, it could go up to 120. The speed of the car depends on the agreed-upon protocol. Common serial protocols include RS-232, RS-422, and RS-485. But what are the key differences between them? Let’s explore together.
First, what exactly is UART?
UART stands for Universal Asynchronous Receiver/Transmitter. It's an asynchronous communication module used for transferring data between devices. Its main function is to convert parallel data into serial format and vice versa. UART defines the frame structure, allowing communication parties to use the same frame format and baud rate to share a clock signal. With just two signal lines (Rx and Tx), it enables asynchronous serial communication, making it widely used in various applications such as mobile phones, industrial controls, and PCs.
By adding a suitable level shifter, like SP3232E or SP3485, UART can support RS-232 or RS-485 communication, or even connect directly to a computer’s port. The versatility of UART makes it a crucial component in many systems.
Asynchronous communication uses a character as the basic unit of transmission. While the time between characters may vary, the interval between bits within the same character remains fixed. The data transfer rate is measured in baud, which refers to the number of bits transmitted per second. For example, if 120 characters are sent each second, and each character consists of 10 bits, the baud rate would be 1200.
The standard data format includes a start bit, data bits, parity bit, and stop bit. These define how data is structured during transmission. Understanding these elements helps in correctly interpreting the received data.
Next, let’s look at the RS-232 standard.
RS-232 is a serial interface standard developed by the Electronic Industry Association (EIA). It specifies electrical and physical characteristics for data transmission. However, it should be noted that RS-232, RS-422, and RS-485 are not communication protocols themselves but rather mechanical and electrical interface standards. Many people mistakenly think they are protocols, which is incorrect.
The RS-232 standard originally used a 25-pin DB-25 connector, but later, IBM reduced it to a DB-9 connector, which became the de facto standard. In industrial control, only three lines are typically used: RXD, TXD, and GND.
UARTs usually have four pins (VCC, GND, RX, TX) with TTL levels—low being 0V and high being 3.3V or higher. However, when connecting to computers, level shifters are often needed.
Now, moving on to the RS-485 and RS-422 standards.
While RS-232 supports point-to-point communication, it doesn't allow networking. To address this limitation, RS-485 was introduced. It uses differential signaling, also known as balanced transmission, which involves a pair of twisted wires labeled A and B. This method enhances noise immunity and allows longer transmission distances.
RS-422 has four signal lines—two for transmission and two for reception—enabling full-duplex communication. In contrast, RS-485 uses only two lines, supporting half-duplex operation, making it ideal for bus networks.
Key features of RS-485 include:
- **Electrical Characteristics**: Logic '1' is represented by a voltage difference of +2 to +6V, and logic '0' by -2 to -6V.
- **Maximum Transmission Rate**: Up to 10 Mbps.
- **Noise Immunity**: Enhanced due to differential signaling.
- **Maximum Communication Distance**: Up to 1200 meters at 9600 bps.
Compared to RS-232, RS-485 offers better noise resistance, longer distance, and multi-node communication capabilities.
Understanding the differences between these standards is essential for choosing the right interface for your application. Whether you're working on industrial automation, embedded systems, or communication projects, knowing how these interfaces work will help you design more reliable and efficient systems.
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