What’s the difference in RS232,RS485 and RS422 protocols?

Many engineers often mistakenly refer to RS-232, RS-422, and RS-485 as communication protocols, but in reality, they are simply mechanical and electrical interface standards for serial communication (at most, they could be considered the physical layer of network protocols). So, what are the differences between them?

Hardware Interface Definitions:

Interface:

• RS232 (Recommended Standard 232): It typically uses a 9-pin or 25-pin connector and is commonly found in serial ports on computers and other devices. RS232 is a single-ended protocol, meaning it uses one signal wire and a ground.
• RS422 (Recommended Standard 422): RS422 uses a 9-pin connector and employs differential signaling with two signal wires, one for transmitting (Tx) and one for receiving (Rx), as well as a shared ground.
• RS485 (Recommended Standard 485): Similar to RS422, RS485 uses differential signaling with two signal wires for data transmission. However, RS485 supports multiple devices on the same bus and employs a multidrop configuration, allowing for more complex network topologies.

Operational Methods:

• RS232: Supports only point-to-point communication.
• RS485: Enables point-to-multipoint communication, where multiple devices can communicate with one another in a master-slave configuration.
• RS422: Similarly supports point-to-multipoint communication.

Communication Modes:

• RS232: Functions as a single-ended system with voltage levels representing logic states.
• RS485: Implements balanced drivers and differential receivers for improved noise immunity.
• RS422: Shares similarities with RS485 but may have stricter impedance requirements.

Logical Characteristics:

• RS232: Logic “1” ranges from -3V to -15V, while logic “0” ranges from +3V to +15V.
• RS485 and RS422: Logic “1” ranges from +2V to +6V, and logic “0” ranges from -2V to -6V.

Resistance to Interference, Transmission Distance, and Transmission Rate:

• RS485 vs. RS232:Interference Resistance: RS485 offers better noise immunity due to its balanced transmission lines.Transmission Distance: RS485 supports longer transmission distances (up to 1200 meters at 9600 bps), whereas RS232 is limited to around 50 meters.Communication Capacity: RS485 allows for up to 128 transceivers on a single bus, facilitating device networking, unlike RS232, which supports only one-to-one communication.Transmission Rate: RS485 achieves higher data rates, up to 10 Mbps, compared to RS232’s maximum of 20 Kbps in asynchronous mode.
• RS422 vs. RS485:RS422: Utilizes 4 signal wires for full-duplex communication, allowing simultaneous transmission and reception. RS485: Employs only 2 data wires for half-duplex communication, where transmission and reception share the same lines and cannot occur simultaneously.

1. RS232: Utilizes a 3-wire full-duplex configuration.
2. RS485: Adopts a 2-wire half-duplex configuration.
3. RS422: Employs a 4-wire full-duplex configuration.

In summary, RS-232, RS-422, and RS-485 differ significantly in hardware interface definitions, operational methods, communication modes, logical characteristics, resistance to interference, transmission distance, and transmission rate, making them suitable for various applications based on specific requirements for distance, speed, and noise immunity.