Serial communication is frequently utilized in systems such as industrial automation and video surveillance, and standards like RS-232 and RS-485 are commonly heard of. So, what exactly is serial communication, and how do these different standards differ from each other?
Serial communication is a widely applied digital communication protocol that enables point-to-point connections between computers and external devices. It transmits data using just one receive line and one transmit line. Devices we use daily, such as printers and projectors, connect to computers through serial communication.
Typical serial communication utilizes three wires for transmission: TX (transmit), RX (receive), and a ground wire. Wiring involves a cross-connection as shown in the diagram, allowing signal transmission between two devices once connected.
The electrical level standard used in this communication method is TTL (Transistor-Transistor Logic). Simply put, it refers to the voltage range during communication. TTL specifies a voltage range of 0-5V, where 0V represents logic 0 and 5V represents logic 1.
However, this narrow voltage range of 0-5V has poor anti-interference capabilities, making it susceptible to external factors that can alter low levels to high, resulting in data errors. Therefore, this method is only suitable for signal transmission on circuit boards or between computers and chips within 1 meter.
To address this issue, the RS-232 standard was introduced. Its interface resembles VGA but with only nine pins. Compared to TTL, RS-232 incorporates a level conversion chip that transforms TTL levels into RS-232 levels, expanding the voltage range and significantly enhancing anti-interference capabilities, enabling transmission distances up to 15 meters.
From TTL to RS-232, the transmission format remains essentially unchanged. But what if longer-distance, large-scale transmission is required? This led to the development of the RS-485 standard. Commonly used in industrial control environments, RS-485 also incorporates a level conversion chip, but it directly converts serial signals into differential signals. Physically, it employs twisted-pair cables, utilizing the difference between signals A and B to represent 0 and 1, eliminating the need for a ground wire.
This approach further enhances resistance to interference, allowing transmission distances up to 1200 meters and higher frequencies. Unlike RS-232, RS-485 is a half-duplex communication protocol, meaning transmission and reception cannot occur simultaneously. This characteristic makes it more suitable for one-to-many communication, enabling a master-slave network connection.