Serial Communication Protocols: RS232 vs. RS485 and Their Applications

What is the RS232 protocol?

The RS232 interface or TIA/EIA-232 is a data transfer protocol designed to control serial communication. It is used to organize the data flowing from a piece of Data Terminal Equipment (DTE) like a terminal or transmitter to a receiver or other type of Data Communications Equipment (DCE).

The distance between the devices directly impacts communication speed. Serial communication can be established in either full-duplex or half-duplex mode. Full-duplex allows data to be sent and received simultaneously by using different wires for transmission and reception. In a half-duplex implementation, a single line is used for both transmitting and receiving data resulting in more limited operation. Half-duplex can be set to transmit or receive data, but not both at the same time.

What is the RS485 protocol?

The most widely used serial interface in industry is the RS485 or EIA-485 protocol. It has a major advantage over RS232 interfaces. Through the use of multi-point topology, multiple receivers and transmitters can be connected. Data transmission is done using differential signals for more consistency.

Two types of RS485 communication are possible:

• RS485 interfaces with 2 contacts operate in half-duplex mode, either sending or receiving data exclusively at one time.
• RS485 interfaces can also have 4 contacts in which case it can run in full-duplex mode. When used in this way data can be transmitted and received simultaneously.

What is the difference between RS232 and RS485 serial ports?

RS232 and RS485 are two standards for electrical transmission that was in existence before the introduction of personal computers. They are employed in interfaces that enable serial communication between computers and peripheral devices. While they both serve the purpose of allowing serial data transmission, the RS232 and RS485 protocols have some differences. Let’s see where these protocols differ.

The main difference between RS232 and RS485 serial ports are in:

- Mode of operation
- Communication distance
- Voltage levels
- The number of contacts in use, etc.

So let’s take a look closer at these differences.

The number of wires - An RS232 cable is typically comprised of 9 different wires, though in some connectors, 25 wires are used. Each wire serves a distinct purpose in the data transmission. RS485 cables only have three wires, 2 for data transmission and 1 for the ground.

Two-way transmission - RS232 is full-duplex, meaning that it can transmit data in both directions, both sending and receiving. To achieve duplex communication with RS485 you need another set of wires. Native RS485 is half-duplex, meaning it can only transmit in one direction at a time.

Voltage - RS485 uses voltages between +5V and -5V. The recommended voltage for RS232 is +12V to -12V, and signals degraded to as low as 3V can still be understood. Range - There are significant differences in the cable lengths that can be used with the RS232 and RS485 protocols. RS232 cables are generally restricted to being 50 feet (15 m ) in length. Cables for RS485 can extend up to 4,000 feet (1,200 m).

The differences in these two protocols can make one solution more favorable than the other in certain applications. The existence of RS232 to RS485 converters allow the two protocols to be used together. These converters usually work in both directions, allowing for both RS232 to RS485 and RS485 to RS232 connections.

Software technology used in serial communication

If you work with serial interfaces and devices you may be in need of some software tools to assist you in carrying out your duties. Electronic Team has developed three applications that make it easy to work with serial devices and extend their capabilities.