Why engineers choose RS-232 over USB

Article By : Jeff Dorsch

The simplicity of RS-232 is a selling point for design engineers, who have to deal with various electronic subsystems that have intricate requirements.

Why is RS-232 being designed into so many products these days, instead of USB or other more contemporary data communication standard?

The RS-232 was introduced in 1962 by the EIA’s Radio Sector as a standard for serial communication between data terminal equipment (such as a computer terminal) and data communication equipment (later redefined as data circuit-terminating equipment), typically a modem. RS-232 serial ports were found on the original IBM Personal Computer and other early desktop PCs. The standard was revised in 1969, then in 1986, and again in 1997 to reflect the changing applications in which it was used, starting with electromechanical teletypewriters and modems, and smart and dumb electronic terminals in the 1960s, PCs and their peripherals in the 1980s, and later PLCs for factories and use in other products.

RS-422 was meant to extend the range of RS-232 connections to up to 1,500m (about 4,900ft). The similar RS-423 standard can cover up to 1,200m (about 3,900ft). RS-422 was used on Apple’s Macintosh desktop computers until 1998, when the iMac came out with a USB connection.

RS-485, which can cover up to 1,220m (about 4,000ft), is used in automation systems (including PLCs), building automation, computer equipment, model railways and theatrical lighting systems.

USB, especially USB 3.0, is significantly faster than RS-232, but it’s also much more complicated. RS-232 operates over short distances, among other disadvantages. USB operates at 5V, while RS-232 can work in a range of up to 15V. The simplicity of RS-232 is a selling point for design engineers, who have to deal with a number of electronic subsystems that have intricate requirements. Making the serial communication of data as simple as possible carries an appeal—almost a retro/vintage feel for technology that predated PCs, mobile devices and wearables.

So, while new standards have appeared in the industry, factors such as cost, speed and applications are ensuring that legacy serial interface standards such as RS-232, RS-422 and RS-485 remain favourite communications technologies for engineers for years to come.

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