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Serial Communication Interfaces:
RS-232 and RS-485

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Today, there are plenty of serial interfaces. Most were developed for specific applications, but some of these interfaces have become common, such as I2S, LIN, CAN, SPI, Flex, MOST, and I2C. There are also higher-speed interfaces you’re probably familiar with like Ethernet, USB, FireWire, HDMI, and Thunderbolt. Two of the oldest interfaces are RS-232 and RS-485. Surprisingly, these legacy interfaces aren’t obsolete. Both are still used in a variety of applications.

This article will provide an overview of the RS-232 and RS-485 serial interfaces and highlight the key differences. Additionally, it will provide some details about USB-C, USB 4.0, and Thunderbolt 3.

What Is the RS-232 Protocol?

RS-232, also known as RS-232C and TIA/EIA-232-E, is a group of electrical, functional, and mechanical specifications for interfaces between computers, terminals, and modems. The RS-232 standard was developed by the EIA (Electrical Industries Association), and it defines requirements for connecting DCE (data communications equipment — modems, converters, etc.) and DTE (data terminal equipment — computers, controllers, etc.) devices.

The standard, which is functionally equivalent to ITU V.24/V.28, specifies the workings of the interface, circuitry, and connector pinning. Both sync and async binary data transmission fall under RS-232. PCs use the RS-232 interface to transmit data to modems, and some printers even use it.

In RS-232 connections, the distance between linked devices affects how quickly data transfers. The shorter the connection, the better the transfer speed.

What Is the RS-485 Protocol?

RS-485 is the most common serial interface utilized today because it offers better functionality than RS-232. This serial interface boasts a greater data rate and range than RS-232. RS-485 also supports multipoint configurations. RS-485’s support for multipoint configurations allows you to link many receivers and transmitters to create a small network of devices.

RS-232 vs. RS-485: Differences and Specific Use Cases

The RS-232 and RS-485 serial interfaces have some major differences. The first difference is the way they transmit data. RS-232 is full duplex, allowing it to send and receive data in two directions. RS-485 is half-duplex, so it can only transmit data one way at a time. RS-485 can perform duplex communication, but, to do so, it requires extra cabling.

The second difference between the two interfaces is their recommended voltage. RS-232 uses voltage between +12V to -12V. RS-485 uses voltage between +5V and -5V.

The third difference is the amount of wiring within each cable. RS-485 cables have 3 wires. One is for the ground and the other 2 send data. The majority of RS-232 cables are made up of 9 wires, but some have up to 25 wires.

The fourth major difference is their data transfer rates. RS-232 supports data rates up to 3 Mbps. RS-485 is much faster and can achieve data rates up to 40 Mbps.

The last main difference between these interfaces is the maximum distance they can extend data. RS-485 cables can extend data 4,000 feet, whereas RS-232 cables can only transmit data 50 feet.

The RS-232 and RS-485 interfaces are not commonly used with computers and devices any longer because of the popularity of interfaces like USB and Thunderbolt. Today, RS-232 and RS-485 are commonplace in industrial monitoring and embedded systems. These interfaces are also used with IoT sensors and monitors and with science and healthcare devices. RS-485 is often even deployed with CCTV cameras.

USB-C, USB 4.0, and Thunderbolt 3

There are many other standard interfaces on the market used to transmit data. Most notable are USB-C, USB 4.0, and Thunderbolt 3.

USB-C is an industry-standard interface for transmitting data and power through one cable. USB-C is capable of supporting data rates up to 40 Gbps – four lanes with 10 Gbps per lane. It can also provide up to 100 watts of power to devices. You can use USB-C to connect and power a variety of devices, including HDMI, MHL, and DisplayPort peripherals.

USB 4.0 is a new interface that should be released in late 2020 or early 2021. Similar to USB-C, it features a 40-Gbps maximum data transfer speed. It can also supply up to 100 watts of power to devices. USB 4.0 will work with the majority of Thunderbolt 3 devices. This new interface is also able to simultaneously transmit video and data better than past USB versions: If you’re transporting video and data at the same time, the USB 4.0 port will manage bandwidth for the best results. USB 4.0 is backward compatible with all previous USB standards.

Thunderbolt is a hardware interface that connects devices to PCs. The latest version of Thunderbolt is Thunderbolt 3. Thunderbolt 3 also supports data transfer speeds up to 40 Gbps. It features USB speeds up to 10 Gbps. This useful interface can connect two 4K monitors, and it supports 10 GbE fast networking, HDMI 2.0, and DisplayPort 1.2.

Which Interface Is Best for My Application?

If you need to make short-distance connections between equipment and are not worried about slower data transmission, use the RS-232 interface. RS-232 supports data rates up to 3 Mbps and can extend data 50 feet. This interface is easy and cheap to set up. Additionally, you can power RS-232 interfaces through one 3.3- or 5-volt power supply.

If you need to make long-distance connections between devices and require higher data transfer rates, choose the RS-485 interface. RS-485 can achieve data rates up to 40 Mbps and can extend data 4,000 feet. RS-485 is also perfect if you want to deploy duplex networking.

If you need to connect your computer or phone to common devices, you are most likely going to use USB or Thunderbolt. Both USB-C and Thunderbolt 3 feature a max data transfer rate of 40 Gbps. This makes both interfaces perfect for connecting multiple displays, high-speed networking equipment, storage devices, docking stations, and more.