Serial data transmission standards

Serial data communications standards - RS-232, RS-422, & RS-485 were widely used for data transfer providing easy connectivity for the time and they can still be found today.


Serial Data Transmission Includes:
Serial data standards    

Common standards:     RS232     RS422     RS449     RS485     20 mA current loop    


Serial data transmission standards including RS232, RS422, RS423, and RS485 were widely used for many data links, proving effective connectivity for the day. Although not nearly as widely used today, they can still be found in some areas.

Serial data transmission links used cables with a variety of different functions to enable data to be sent reliably between two equipments. Although a number of different wires were often used within a cable, they did not require nearly as many as the systems that sent parallel data.

Serial data standards background

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.

Serial data standards summary

  • RS232   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.   . . . Read more about RS232.
  • RS422   This standard gives a much higher data rate than RS232, but it uses differential transmission techniques. Many RS422 devices are compatible with RS232.   . . . Read more about RS422.
  • RS485  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.   . . . Read more about RS485.

Comparison of serial data standards


Common RS Series Serial Data Transmission Standards
 
Parameter RS232 RS422 RS485
Cabling Single ended Differential Differential
Number of devices One transmit and one receive Five transmitters and ten receivers 32 transmitters and 32 receivers
Communication mode Full duplex Full duplex / half duplex Half duplex
Maximum distance 50 feet at 19.2 kbps 4000 feet at 100 kbps 4000 feet at 100 kbps
Maximum data rate 19.2 kbps at 50 feet 10 Mbps at 50 feet 10 Mbps at 50 feet
Signalling mode Unbalanced Balanced Balanced
Mark (1) -5 to -15 V 2 V to 6 V max. (B>A) 1.5 V to 5 V max. (B>A)
Space (0) +5 to +15 V 2 V to 6 V max. (A>B) 1.5 V to 5 V max. (A>B
Output current capability 500 mA 150 mA 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.



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