Older Communication Standards Still Compete with USB

By Jeff Dorsch

Why is RS-232 being designed into so many products these days, instead of Universal Serial Bus (USB) or another, more contemporary data communication standard? Ask a dozen engineers, and you're likely to get up to a dozen different answers.

A consensus answer could be that RS-232 is familiar to many EEs. The standard dates back more than five decades, and it is also known as EIA RS-232, EIA 232, and TIA 232 (for the Electronic Industries Alliance and the Telecommunications Industry Association, respectively).

Search the Mouser Electronics website for RS-232 products and you'll find some 5,750 results, in the categories of circuit protection, connectors, electromechanical, embedded solutions, optoelectronics, passive components, power, semiconductors, sensors, test and measurement, thermal management, tools and supplies, and wire and cable.

And it's not just the ancient (from the electronics industry's perspective) RS-232 specification that has a long history. There's also RS-422, RS-423, RS-485 (16 years old), I2C (32 years old), Serial Peripheral Interface (SPI), and more.

In contrast, USB is a relative youngster in communications specifications. Seven companies began developing USB in 1994. (Three of those companies - Compaq Computer, Digital Equipment, and Nortel Networks - no longer exist, although their technologies are still in use throughout the industry.) The USB 1.0 specification was first published in January 1996, and the subsequent USB 1.1 version gained wider adoption of the technology. The USB 2.0 and USB 3.0 releases followed in 2000 and in 2008, respectively.

History of the Serial Interface Standard

RS-232 (the "RS" stands for "recommended standard") 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 programmable logic controllers (PLCs) for factories and use in other products.

Figure 1: Analog Devices' fully isolated single-package surface-mount RS-232 transceiver with integrated dc/dc converter to supply isolated power. (Source: Analog Devices)

RS-422 was meant to extend the range of RS-232 connections to up to 1,500 m (about 4,900 ft). The similar RS-423 standard can cover up to 1,200 m (about 3,900 ft). 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 1,220 m (about 4,000 ft), is used in automation systems (including PLCs), building automation, computer equipment, model railways, and theatrical lighting systems.

Why Design Engineers Still Use RS-232

So, with advances in technology, why are some engineers still selecting RS-232 instead of USB? One reason is cost considerations. If a company is not a member of the USB Implementers Forum (USB-IF), the USB industry organization, the USB logo trademark license fee is $3,500 for two years, and purchasing a vendor identification from USB-IF is $5,000. No licensing fees or organization dues are needed for implementing RS-232.

Of course, applications and speed are also key considerations in choosing RS-232 and similar specifications.

"Despite the development of newer digital interface standards, the humble RS-232 serial port is still a very popular means of data transfer. Robust and easy to use, the RS-232 interface is still an attractive alternative to the more demanding and temperamental digital interfaces," Maxim Integrated notes on its website.

Figure 2: The MAX220, MAX232, and MAX232A pin configuration and typical operating circuit. (Source: Maxim Integrated)

The RS-232 specification can support data rates of up to 920 kbits/s. An RS-232 is a point-to- point connection made between a Data Terminal Equipment (DTE) device and a Data Communications Equipment (DCE) device. One major limitation of the RS-232 is its maximum cable length of 50 ft, but this can usually be overcome by the transmission of serial data over another medium. At 10 Mbits/s, the RS-422 provides an incredibly fast serial data rate in a multidrop configuration, allowing for up to 10 unit loads. Its use of voltage differences makes it ideal for noisy environments. The RS-485 specification also allows for data rates up to 10 Mbits/s, but it is different from the RS-422 in that it offers a multipoint configuration, allowing for support of multiple drivers and multiple receivers. And, due to its bidirectional interface, the RS-485 can support up to 32 unit loads.

RS-232 finds applications today in embedded electronics, industrial control systems, networking equipment, and scientific instruments, among other uses. USB interfaces dominate in personal computers and mobile devices, yet serial ports - which once were prevalent in early PCs - are still finding use in modern electronics. The whole Internet of Things scheme calls for technology that is not only low in power requirements, but also simple to design and implement. Millions, if not billions, of sensors and sensor hubs will be tied to the Internet and those connections must be consistent and reliable.

RS-232 technology has been embraced in the Arduino community, a collection of artists, designers, enthusiasts, hobbyists, and others who make use of the Arduino programming language, development environment, and open-source code. Pre-assembled Arduino boards are available from Mouser.com. Arduino offers free hardware reference design kits that can be downloaded for those who want to assemble their own single-sided boards, and a tutorial on communicating with a computer employing a MAX3323 single-channel RS-232 driver/receiver chip (or a similar semiconductor device) and a software serial connection on the Arduino board. The tutorial offers step-by-step instructions on preparing the breadboard and writing a simple program for the serial data communication.

The Arduino Forum hosts multiple discussions concerning RS-232, such as using RS-232 to control laboratory equipment, connecting with a GPS sensor, communicating with medical equipment, communicating with a GSM TC35 modem module, and controlling a television set. A forum inquiry on "Arduino with RS232 connection" in May quickly attracted more than a dozen responses from other engineers working with the open-source Arduino technology. To connect Arduino boards with most contemporary PCs, an RS-232-to-USB adapter cable is often required, and not all such adapters work with the multiple versions of RS-232. The forum posts note that the cables have drivers that may not work with Windows 7 or Windows 8 (especially Windows 8), making that an important consideration when working with Arduino boards and most PCs made in recent years, which have USB connections and not RS-232.

The RS-422 and RS-485 standards offer some advantages over RS-232, in terms of the distances for sending signals and data transmission speeds. Providing even more advantages and features are the I²C and SPI specifications. I²C was originally developed as Inter-Integrated Circuit by the Philips Semiconductors unit of Royal Philips Electronics, a chipmaker now known as NXP Semiconductors, which spun off from Philips in 2006. Other semiconductor manufacturers embraced the protocol in the 1990s, such as Intersil, Motorola Semiconductor Products Sector (now Freescale Semiconductor), NEC, Siemens Semiconductor (now Infineon Technologies), SGS-Thomson (now STMicroelectronics), and Texas Instruments. Intel defined the System Management Bus (also known as SMBus or SMB) in 1995 as a subset of the I²C serial bus protocol. SMBus has a variant called Power Management Bus or PMBus. Licensing fees for implementing I²C ended in 2006, although NXP Semiconductors continues to collect fees for I²C slave addresses. SPI builds on previous serial interfaces, offering higher throughput than I²C or SMBus, but it has some disadvantages, such as requiring more pins on an IC package, compared with I²C.

Communications Interfaces - Past, Present and Future

There are, of course, many other types of communications interfaces, some of which have come and gone over the years, such as FireWire, the Apple-developed serial bus interface. In addition to USB, there's Ethernet, Fibre Channel, InfiniBand, Musical Instrument Digital Interface (MIDI), Peripheral Component Interconnect and PCI Express, Small Computer System Interface (followed by Parallel SCSI and Serial Attached SCSI), and Advanced Technology Attachment (followed by Parallel ATA and Serial ATA), among others.

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 5 V, while RS-232 can work in a range of up to 15 V. 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 wearable gadgets.

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 favorite communications technologies for engineers for years to come.

Jeff Dorsch is a freelance editor, researcher, and writer based in the San Francisco Bay Area. He served as editor-in-chief of Electronic News, a weekly trade publication, and has more than three decades of experience in covering technology.