RS232, RS422, and RS485 are serial data communication protocols. Their main differences are the topic of this post.
Why these standards were created?
They were created for the efficient and structured transfer of data between devices. Thanks to these standards, digital communication between devices from different manufacturers is possible. Serial communication protocols transmit data through a single medium, unlike parallel communication, where each bit of data requires a separate wire for transmission.
Some important concepts
Types of data transfer
Full duplex is the most common type, in which equipment can transmit and receive data at the same time.
Topology
RS232
Introduced in the 1960s by the Electronic Industries Association (EIA), it is the oldest of the three protocols. It was used for a long time in communication between computers and peripherals, such as printers, modems, and scanners. Data transmission in RS232 is full duplex, and the topology is point-to-point. Although USB has replaced RS232 in many applications, this protocol is still used in some industrial control systems, medical equipment, and older computers due to its simplicity and low power consumption.
Electric features
The maximum range is 15 meters (49 feet), and the data transfer rate is typically 19.2 kbps (kilobits per second), but it can reach up to 115.2 kbps. The data transfer type is full duplex, and the topology is point-to-point. However, only a single transmitter and a single receiver can be connected, and RS232 is susceptible to interference.
RS232 terminals and its functions
- SG: ground terminal.
- DTR: indicates that the start between equipment can begin.
- TD: data transmission.
- RD: data reception.
- DCD: carrier detector, a carrier is a high frequency signal which transports information through modulation.
- RI: sinalizes a received call.
- CTS: a device sends a signal through this terminal, to indicate that it’s ready to receive data.
- RTS: line to initiate data transfer.
- DSR: sinalizes that it can set a connection.
RS422
Developed in 1975, to increase the transmission rate and range, RS422 can reach up to 1200 meters (3937 ft), while the speed depends on the distance and can range from 100 kbps to 10 Mbps (megabits per second), with the higher speed achieved at shorter distances. In contrast to RS232, it is possible to connect 1 transmitting device to up to 10 receiving devices.
RS422 also adopts point to point topology and full duplex transfer mode.
RS485
In RS422, receiving devices cannot transmit data on a shared channel. To solve this problem, the RS485 protocol was created, in 1983. In addition to having a multipoint topology, meaning multiple transmitters and receivers can be connected to the same channel, it also uses differential signaling and has the same range as RS422.
With RS485, you can connect up to 32 devices, employing repeaters and signal amplifiers, the maximum device number can reach to 256. The speed is the same of RS422.
