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 confuse UART with RS-232, RS-422, and RS-485, not understanding their connections or differences. This article aims to clarify these concepts and help you better understand how they relate to each other.
If we compare serial communication to traffic, then UART can be thought of as a station. A single frame of data is like a car traveling on the road, following specific rules. In a city, the speed limit might be 30 or 40 km/h, while on the highway, it could go up to 120 km/h. The speed of the car depends on the agreement set by the protocol. Common serial protocols include RS-232, RS-422, and RS-485. What are the subtle differences between them? Let’s explore together.
First, what is UART?
UART stands for Universal Asynchronous Receiver/Transmitter. It is an asynchronous transceiver used for communication between devices. The main function of UART is to convert data between serial and parallel formats. It defines the frame format, allowing both parties to use the same format and baud rate to share a clock signal. Communication can be done using just two signal lines (Rx and Tx), making it known as asynchronous serial communication.
Adding a suitable level shifter, such as SP3232E or SP3485, allows UART to be used in RS-232 or RS-485 communication, or even connected to a computer's port. UART is widely used in applications like mobile phones, industrial control systems, and PCs.
UART uses asynchronous, serial communication. In serial communication, data is transmitted bit by bit over a single line. This method is simple, cost-effective, and suitable for long-distance communication, although it has a slower transmission speed.
Asynchronous communication sends data in characters. The time between two characters is variable, but the interval between bits within a character is fixed. Data transfer rate is measured in baud rate, which is the number of bits per second. For example, if the data transfer rate is 120 characters per second, and each character is 10 bits, the baud rate would be 1200.
The data communication format includes a start bit, data bits, parity bit, and stop bit. The start bit signals the beginning of a character, followed by the data bits, which can be 5 to 8 bits. A parity bit checks for errors, and the stop bit marks the end of the character.
It’s important to note that asynchronous communication relies on synchronization within each character. Once the start bit is detected, the receiver can correctly receive data by maintaining synchronization with the transmitter during that time. The next character’s start bit will reset the synchronization.
Second, the RS-232 standard.
RS-232 is a serial physical interface standard developed by the Electronic Industry Association (EIA). "RS" stands for "Recommended Standard," and "232" is the identification number. RS-232 specifies the electrical and physical characteristics of the data transmission path. However, it does not define how data is processed. Many people mistakenly believe RS-232, RS-422, and RS-485 are communication protocols, but they are actually only mechanical and electrical interface standards for UART communication (or at most, the physical layer of a network protocol).
The standard originally specified a 25-pin DB-25 connector, defining the signal content of each pin and its voltage levels. Later, IBM reduced this to a DB-9 connector, which became the de facto standard today. In industrial control systems, the RS-232 port typically uses three lines: RXD (2), TXD (3), and GND (5).
In the past, since PCs had an RS-232 interface, many chose it when working with UARTs. However, modern computers no longer have an RS-232 interface. Instead, development boards often use TTL UART or directly connect UART to USB.
Embedded serial ports usually refer to UART ports, but many don’t distinguish between them and COM ports, or between RS-232 and TTL. In fact, UART and COM refer to the physical interface form (hardware), while TTL and RS-232 refer to the electrical signal levels (voltage levels).
UART has four pins: VCC, GND, RX, and TX. It operates at TTL levels, where a low level is 0V and a high level is 3.3V or higher.
Third, the RS-485 and RS-422 standards.
While RS-232 enables point-to-point communication, it doesn’t support networking. To solve this, RS-485 was introduced. RS-485 uses differential transmission, also known as balanced transmission, and employs a pair of twisted wires—A and B.
Normally, the voltage difference between A and B is +2 to +6V for logic "1" and -2 to -6V for logic "0." There is also a signal ground C, and RS-485 has an "enable" terminal, unlike RS-422.
The electrical performance of RS-422 is similar to RS-485. The main difference is that RS-422 has four signal lines: two for transmitting and two for receiving. Since the transmit and receive channels are separate, RS-422 supports full-duplex communication and is suitable for communication between two stations. However, it cannot support bus networks. RS-485, with only two signal lines, works in half-duplex mode and is commonly used in bus networks.
RS-485 has several advantages:
1. Electrical characteristics: Logic "1" is represented by a voltage difference of +(2~6)V between two lines, and logic "0" is -(2~6)V.
2. Maximum data transfer rate: Up to 10 Mbps.
3. Anti-noise capability: Enhanced due to differential signaling.
4. Maximum communication distance: About 1219 meters, with the distance inversely proportional to the data rate. At 100Kbps, the maximum distance is achieved. For longer distances, a 485 repeater is required. A typical RS-485 bus supports up to 32 nodes, and with special chips, it can reach 128 or 256 nodes.
Despite its early adoption, RS-232 has several limitations:
1. High signal levels, making it prone to damaging interface circuits.
2. Low transmission rate, especially in asynchronous mode (20Kbps).
3. Common-mode interference due to a shared ground.
4. Limited transmission distance (about 50 meters).
5. Only supports one-to-one communication, not forming a serial bus.
Unbalanced serial communication interfaces like RS-423 and RS-449 exist, but balanced interfaces like RS-422 offer better noise immunity and longer distances.
RS-422 (EIA RS-422-A) is a serial port standard used in Apple Macintosh computers. It uses differential signals, making it more resistant to noise than RS-232.
Fourth, comparing RS-232 and RS-485:
1. Anti-interference: RS-485 uses differential signaling, providing better noise immunity.
2. Transmission distance: RS-485 can go up to 1200 meters, while RS-232 is limited to about 15 meters.
3. Communication capability: RS-485 supports up to 128 nodes, while RS-232 is limited to one-to-one.
4. Transmission rate: RS-232 is slow (20Kbps), while RS-485 can reach 10Mbps.
5. Signal lines: RS-485 requires two lines for half-duplex, while RS-232 typically uses three lines.
6. Electrical levels: RS-485 uses voltage differences between two lines, while RS-232 uses absolute voltages.
Comparing RS-422 and RS-485:
1. RS-422 has four signal lines (two for transmit, two for receive), supporting full-duplex communication.
2. RS-485 has only two lines, supporting half-duplex communication.
RS-485 is widely used in industrial applications due to its long-distance capability, multi-node support, and low cost.
Factors affecting RS-485 communication reliability include signal reflection, signal attenuation, and distributed capacitance. Proper termination resistors and biasing can help mitigate these issues.
In summary, serial ports are common device interfaces used in instrumentation and remote data collection. They are easy to develop and popular among engineers. Understanding the differences between UART, RS-232, RS-422, and RS-485 is essential for designing reliable communication systems.
HP Probook 440 445 G8,M23770-B31,M23769-001,M23770-001
S-yuan Electronic Technology Limited , https://www.syuanelectronic.com