Design Con 2015

The Serial Port: It Just Keeps Going

-September 17, 2013

Just because new computers no longer include the venerable serial port doesn't mean it's dead. In fact, the serial port is still thriving in many test instruments, industrial controls and medical devices. You'll find RS-232, RS-422, and RS-485 serial ports on millions of devices in use today.

With new PCs lacking serial ports, you need some kind of converter board to connect to your equipment. There's a myriad of converter boards and modules out there, from PCI Express and USB to legacy fiber and the ISA bus.

Serial-port converters range in price starting as low as $12 (Figure 1) and are available through Amazon, Newegg, and many other consumer and industrial distributors.


Figure 1. This PCI to single RS-232 post convert costs just $12.

At the high end, some converter cards cost over $1,200, such as the NI PCIe-8430/16 16-port PCIe adapter from National Instruments. High-end serial-port converters offer high-speed access to system memory, software adjustable data rates, and multithreading, which means they’re not interrupt driven like lower-end converters. The number of serial ports on a converter can span from one to 16. Some converters provide optically isolated ports that eliminate ground loops, which often create problems in industrial applications.

Many serial-port converters connect through cables that run between the converter card and your equipment.

Figure 2. A Sealevel Systems PCIe converter provides eight serial ports through a common cable from the board's 78-pin connector.

Some serial converters are housed in boxes rather than as cards for desktop computers. These configurations let you use laptop computers to communicate with your equipment. Most converter boxes connect to the host computer over a USB link. While most box-type serial converters use 9-pin D-sub connectors, some, such as the QSU2-540IS from Quatech (now B&B Electronics), connect to industrial devices through a terminal strip.


Figure 3. For industrial applications, you can connect devices to this
Quatech serial-port converter through a terminal strip.


When I first started using RS-232, the top speed available was 9,600 bits/s, and most links ran slower, down to 1,200 bits/s. I recall having to drop the speed of a link from 9,600 bits/s to 2,400 bit/s to get reliable communication over a ribbon cable. Crosstalk was the problem. Some of today's RS-232 links run at speeds up to 921.6 kbits/s. Such speeds were unheard of not long ago.

Do you use serial ports in test, home, medical, or industrial applications? Tell us about your application.

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