What is the difference between RS232, RS422 and RS485?

Photocoupler

1. RS-232-C

RS-232-C is a serial physical interface standard developed by the Electronic Industry Association (EIA). The term "RS" stands for "Recommended Standard," with "232" being the identifier and "C" indicating the revision level. This standard defines 25 signal lines, including one main channel and one auxiliary channel. In most cases, only the main channel is used.

For general full-duplex communication, only a few lines are typically required: one transmit line, one receive line, and one ground line. The data transmission rate specified by the RS-232-C standard ranges from 50 to 19200 baud per second, offering flexibility for different applications.

According to the RS-232-C standard, the driver can handle up to a 2500pF capacitive load. This capacitance limits the communication distance. For example, using a 150pF/m cable, the maximum distance is about 15 meters. The short transmission range is also due to the fact that RS-232 uses single-ended signaling, which is prone to common-mode noise and cannot effectively suppress interference. As a result, it is commonly used for short-range communication, typically within 20 meters.

2. RS-485

RS-485 is widely used when longer communication distances are required—ranging from tens of meters to several kilometers. It uses balanced transmission and differential reception, making it highly resistant to common-mode interference. The high sensitivity of the bus transceiver allows it to detect voltages as low as 200mV, enabling reliable signal recovery over long distances.

RS-485 operates in half-duplex mode, meaning only one device can transmit at a time. Therefore, the transmitting circuit must be controlled by an enable signal. This protocol is ideal for multi-point networks, as it can support up to 32 drivers and 32 receivers connected in parallel, making it suitable for distributed systems.

3. RS-422

RS-422 shares a similar circuit design with RS-485, using differential signaling for both sending and receiving. Unlike RS-232, it does not require a digital ground, which reduces complexity. The differential operation is what allows for longer transmission distances under the same data rate conditions.

RS-422 supports full-duplex communication using two pairs of twisted wires, allowing simultaneous transmission and reception. In contrast, RS-485 is limited to half-duplex operation, where transmission and reception cannot occur simultaneously, but it only requires one pair of twisted wires. Both standards can transmit up to 1200 meters at 19.2 kbps.

Understanding these differences helps in selecting the appropriate interface for specific communication needs. Whether it's short-range, cost-effective RS-232, or long-distance, robust RS-485 and RS-422, each has its own advantages depending on the application.

(Editor: Circuit Diagram)