Serial data transmission standards

- overview of the standards for serial data communications including RS232 (RS-232), RS422 (RS-422), RS423 (RS-423), and RS485 (RS-485)

Serial data formats for the transmission of data, including RS232, RS422, RS423, and RS485 find many uses within data communications and networking. Although serial formats are not as fast as parallel formats, they are often more suitable because the cables require less constituent wires and as a result they are much cheaper and more flexible than their parallel equivalents. This enables them to be used for longer cable runs and in areas where thicker cables would not be applicable.

The first of the RS standards was RS232, or more correctly RS-232. This was developed in 1962 when the need for forms of transmitting data from modems attached telephone lines to remote communications equipments became apparent. The 'RS' stands for Recommended Standard, although later these standards were formally adopted by the EIA / TIA in the USA. The EIA is the Electrical Industries Association and the TIA is the Telecommunications Industries Association. Once RS-232 was established an equivalent standard was written for the ITU (International Telecommunications Union) to provide a more international standard. This would enable the same standards to be used worldwide and also give manufacturers access to a global market using just one product. This standard was known as V.24 and is totally compatible with RS-232.

With RS232 well established and the need for faster communications and over longer distances, further standards beyond RS232 were introduced. Although a number of standards were introduced, the most widely used are RS-422 and RS485.

RS232 summary

RS-232 is the most widely used serial standard that is in use. Many laptop computers incorporate a serial interface, and it was also used on many printers, although much less so now.

• Cabling - single ended
• Number of devices - one transmit and one receive
• Communication mode - full duplex
• Maximum distance - 50 feet at 19.2 kbps
• Maximum data rate - 19.2 kbps at 50 feet
• Signalling mode - unbalanced
• Mark (1) - -5 to -15 V
• Space (0) - +5 to +15 V
• Output current capability - 500 mA

RS422 summary

This standard gives a much higher data rate than RS232, but it uses differential transmission techniques. Many RS422 devices are compatible with RS232.

• Cabling - differential
• Number of devices - five transmitters and ten receivers
• Communication mode - full duplex / half duplex
• Maximum distance - 4000 feet at 100 kbps
• Maximum data rate - 10 Mbps at 50 feet
• Signalling mode - balanced
• Mark (1) - 2 V to 6 V max. (B>A)
• Space (0) - 2 V to 6 V max. (A>B)
• Output current capability - 150 mA

RS485 summary

RS-485 is a standard that allows high speed data transmission along with multiple transmitters and receivers and this makes it able to be incorporated as a network solution.

• Cabling - differential
• Number of devices - 32 transmitters and 32 receivers
• Communication mode - half duplex
• Maximum distance - 4000 feet at 100 kbps
• Maximum data rate - 10 Mbps at 50 feet
• Signalling - balanced
• Mark (1) - 1.5 V to 5 V max. (B>A)
• Space (0) - 1.5 V to 5 V max. (A>B)
• Output current capability - 250 mA

Each of these standards meets a different requirement. RS232 still being very widely used despite the fact that it has been in use for over 40 years. However the other standards that have been introduced more recently provide higher levels of performance that are very useful in may applications.

Further information on these and other standards can be access through the Telecommunications and networking homepage.