RS232 vs RS422 vs RS485: Serial Interface Differences

This article explains the differences between RS232, RS422, and RS485 serial interfaces, all commonly used for short-distance data transmission, particularly in monitoring and control applications. Let’s dive into each interface:

RS232 Interface

Before USB and PS2 became commonplace, RS232 was the go-to interface for connecting devices like mice, modems, and printers to computers.

RS232 is a single-ended serial interface, meaning it has one wire for transmitting data and another for receiving. This setup allows for a direct connection between one transmitter and one receiver.

For DB9 connectors, pin 2 is RxD (Receive Data) and pin 3 is TxD (Transmit Data). To connect two devices, you’ll need a cable with the following cross-connections:

(DB9) <-------->(DB9)
2(RxD)<-------->3(TxD)
3(TxD)<-------->2(RxD)

With DB25 connectors, pin 2 is TD (Transmit Data) and pin 3 is RD (Receive Data). The wiring for connecting RS232 DB9 to DB25 looks like this:

(DB9) <-------->(DB25)
2(RxD)<-------->2(TD)
3(TxD)<-------->3(RD)

RS232 uses full-duplex transmission, allowing data to be sent and received simultaneously. It can achieve data rates up to 1 Mbps over distances of around 50 feet.

RS232 Interface Features:

• Cabling Technique: Single-ended
• Communication Mode: Full-duplex
• Number of Devices: 1 transmitter and 1 receiver
• Maximum Distance: 50 feet at 19.2 Kbps
• Maximum Data Rate: 1 Mbps up to 50 feet
• Mark (Data ‘1’): -5V min. and -15V max.
• Space (Data ‘0’): 5V min. 15V max.
• Signaling: Unbalanced

You can find more details on RS232 interface pin diagrams for DB9 and DB25 connectors online.

RS422 Interface

The RS422 interface is designed for faster data transmission over longer distances compared to RS232. It achieves this through differential transmission, utilizing twisted-pair cables to minimize noise.

Instead of using a single voltage level, RS422 represents binary ones and zeros using the voltage difference between two lines.

In an RS422 setup, one master device can control up to 10 slave devices. This interface can handle data rates of 10 Mbps for distances up to 50 feet, and 100 Kbps for distances up to 4000 feet.

RS422 Interface Features:

• Cabling Technique: Single-ended multidrop configuration
• Communication Mode: Full and half-duplex
• Number of Devices: 1 transmitter, 10 receivers
• Maximum Distance: 4000 feet (at 100 Kbps)
• Maximum Data Rate: 10 Mbps (at 50 feet)
• Mark (Data ‘1’): 2V min. (cable ‘B’ > cable ‘A’), 6V max. (A > B)
• Space (Data ‘0’): 2V min.(A > B), 6V max.(A > B)
• Signaling: Balanced

You can find more on RS422 interface pin diagrams with a quick search.

RS485 Interface

RS485 can be considered an upgraded version of RS422. It expands the capabilities of RS422, allowing one device to control up to 32 devices. In many ways, it’s similar to RS422.

RS485 Interface Features:

• Cabling Technique: Multidrop
• Communication Mode: Half-duplex and full-duplex
• Number of Devices: 32 transmitters, 32 receivers
• Maximum Distance: 4000 feet (at 100 Kbps)
• Maximum Data Rate: 10 Mbps (up to 50 feet)
• Mark (Data ‘1’): 1.5V max.(B>A)
• Space (Data ‘0’): 1.5V max.(A>B)
• Signaling: Balanced

You can find more information on RS485 interface pin diagrams online.