The 10 Commandments of USB
The increasing complexity of and demands on networks may be solved with USB technology, but to simplify installations, be sure to follow these 10 rules.
As an industrial, medical, laboratory, or field services interface, USB has many benefits that outweigh the risks. However, you can reduce your odds of disaster with simple fixes – like high retention ports and isolation.
To simplify things, just follow these 10 rules:
I. Thou shalt not allow thy USB cabling to be cast loose upon the ground, lest thee be consumed by fire.
One of USB’s great advantages is that it provides a 5V power supply on a single wire. Connected USB devices can then be powered by the USB cable connection, which eliminates the need for an external power supply. In safe office environments it’s very convenient and normally quite harmless.
Unfortunately, USB was also designed for quick and easy connections/disconnections. That becomes a problem when USB migrates off the desktop and out into the real world. Standard USB connectors don’t grip cables firmly enough to withstand the heavy vibration encountered in industrial applications. The cables can work themselves loose, and when they do, there’s a risk of arcing. In the wrong environment that can lead to fire or explosion.
Loose USB cables can also lead to data loss, software hang-ups, and blue screens.
So be wise when using USB off the desktop. Deploy devices equipped with high-retention USB ports, such as those that require 3.2 lbs of force to dislodge them (lest thee regret the error of thy ways only after it be too late).
II. Thou shalt not assume that 500mA dwelleth within every USB port, for as is so cryptically written: “Assume maketh a donkey out of thee and me.”
According to the USB specification, a device should be able to draw up to 5 unit loads from a USB port. (A unit load in USB 2.0 is defined as 100mA.) “Low-power” devices should be able to draw one unit load or less (under 100mA); “high-power” devices should be able to draw the full 500mA. Self-powered devices like printers typically register as low-power devices, requiring only 100mA.
Perhaps because so many USB devices are self-powered, some manufacturers cut corners and build boards that don’t provide the full 500mA to the USB ports. It may seem as if the connected device or the connection itself is to blame. You’ll try swapping out the USB cables, you’ll try reinstalling software, you’ll be frustrated, and eventually you’ll figure out that the connected device was never receiving full power. After wasting all that time on the problem, you may feel like a donkey.
You’ll feel better after you’ve installed a powered USB hub or powered USB isolator. Powered USB devices restore the full 500mA.
III. Thou shalt not cast aside thy legacy data devices, for USB can be made to speak unto them, and they unto USB.
It’s hard to find a computer with a serial port these days; the USB bus has become the standard. But there is still a huge installed base of legacy serial equipment out there, and there are numerous new applications for which the serial communications protocol is still the perfect answer. As they say, “If it be not broke, thou ought not fix it.”
Rather than abandon serial communications prematurely, the wiser and easier move is to deploy USB-to-serial converters. The serial protocol may have been devised long before USB was even on the horizon, but that doesn’t mean that your data communications can’t flow smoothly over both.
IV. Thou shalt note that even the journey of 20,000 leagues beginneth with the first few cubits.
USB has an effective range of about 11 cubits (5 meters). So what happens if you want to communicate with a device that’s in another building, on the other side of a river, or many miles away? Every communications problem calls for a slightly different solution, be it long-range wireless bridges, fiber optics, copper wire range extension, or serial cable. And eventually all of those data communications protocols will need to cover the last few cubits that lead to your USB port. No worries. You can deploy devices that will convert USB to serial, Wi-Fi, Ethernet, fiber optic, or Modbus with no interruption of your data stream, and they’re easy to use.
V. Thou shalt not be profligate with thy gold when good copper lies before thy very eyes.
Don’t overlook easy opportunities to save money when designing your communications network. You very well may discover that significant savings lie right under your nose, in the form of legacy cabling like coaxial cable and telephone wiring. All kinds of possibilities exist. For example, you could convert a USB signal to an Ethernet package and then use a pair of Ethernet extenders to send your data up to 8,530 feet (2,600m) via coaxial cable or up to 6,200 feet (1,900m) over a pair of unused straight copper wires. That old copper cabling may be worth some money down at the recycling center, but it’s probably worth a lot more if it stays right where it is.
VI. Thou shalt fear the righteous lightning, for it goeth where it darn well pleaseth.
Don’t forget that USB uses copper wire and can thus transfer power surges from lightning strikes or heavy machinery to places where they can do serious damage. You can prevent this by using USB isolators, which can be standalone devices or incorporated into USB hubs, USB serial converters, and other USB devices.
VII. Thou shalt remember the forces unseen, lest magnetism bite thee in the posterior.
Standard USB cabling and equipment isn’t intended for use around magnetic fields. Just being placed in a medical cart, the crowded trunk of an emergency vehicle, or a tight panel may be enough to cause data loss and software glitches when electromagnetic interference from other devices (EMI) induces current in the USB cable. Fortunately, both USB devices and USB cable are available with EMI shielding.
VIII. Thou shalt also consider thy ground potentials when USB strayeth from the desktop.
Thanks to media converters, there’s no reason you can’t connect to remote devices via your USB port. But the farther away that device is, the more likely it will get its power from a different building ground reference. If that’s the case, the USB cable’s ground wire can create a ground loop path. The USB specification more or less assumes that connected devices will be grouped closely together, with a computer and its peripherals sharing a wall outlet and a common ground. In industrial applications, however, the connected devices may not even be in the same building, much less share a single wall outlet. A process control system might be powered from one source, the front panel might be powered somewhere else, and connecting your PC to the front panel via USB can then create a ground loop with the remote process control system. If you’re lucky, the only result will be software hang-ups and blue screens. If you’re not, surges and voltage overloads can burn out integrated circuits and connectors.
The potential for ground loops can be controlled with USB isolators (allowing thy USB applications to stray from the desktop with righteous impunity).
IX. Thou shalt remember only the daft deploy multiple devices where one sufficeth.
As networks continue to expand in both complexity and sheer size, it becomes increasingly likely that yours will employ a variety of protocols: USB, Ethernet, WiFi, serial, or Modbus. And the connecting media may vary as well, from USB cable to Cat 5, and from fiber optic to no wire at all. You may find yourself deploying many different kinds of converters as well as many different kinds of hubs and isolators. One way to simplify matters is to use equipment that provides multiple functions within the same device. Don’t just install a USB hub; install an isolated USB hub. Don’t just deploy a USB-to-serial converter; deploy an isolated USB-to-serial converter. Simplify things.
X. Thou shalt not be farthing wise and pound foolish when designing thine installations.
As has already been said, the USB specification was intended for use in climate-controlled homes and offices, but it has proven to be so useful that USB is now used in everything from hospitals to heavy industry. There’s a good chance that you’ll want to use USB in some difficult environments, and the standard cables and devices that you can pick up at your local office store may not be able to stand up to it. Repair work is expensive, reprogramming is expensive, and downtime is usually unacceptable. If you ruggedize your installation with industrial-grade equipment right from the beginning, it’s likely that you’ll recoup the extra investment very quickly. (And then thou mayest pat thyself on thy back for thy wisdom, thy foresight, and a job well done the first time.)
This article was contributed by B+B SmartWorx.