Ruggedizing USB Connections for Tough Environments
When technician John Baker of Premier Resource Services connected his laptop to a customer’s industrial machinery he got a nasty surprise. There was a quick puff of smoke and John lost his hard drive, his USB port and the USB adapter that he had connected to the machine.
The problem, the technician soon discovered, was that, while his laptop was plugged into the wall, the machine he was servicing was connected to the shop’s single-phase power supply and its leads had been reversed to keep it from tripping a GFI switch. As the laptop and the connected device now had different ground potentials, the USB cable became the path to a lower ground state. It was an expensive lesson.
A kiosk manufacturer plugged a card reader into the kiosk’s built-in computer. Everything was powered up and all of the connections looked fine, yet the two devices would not communicate. The natural inclination was to assume that either the card reader or the cable was faulty. But that was not the issue. The problem lay in the kiosk’s built-in computer. It was a low cost, single-board device and its USB ports were not providing full power.
An injection molding machine manufacturer found a way to enhance parts quality and cut material costs by regulating the temperatures of multiple heating elements. The process created a powerful electromagnetic field. The controlling computer could be kept at a safe distance, but the USB hub and cables were inside the field, and the resulting electromagnetic interference was a communications nightmare.
“You can expect all kinds of problems when you’re working with USB,” says Mike Fahrion, Director of Product
A Laundry List of Inherent Weaknesses
USB was intended to connect mild-mannered home and office devices like printers, keyboards, media players, cameras and external disk drives. And, it would not just connect them; it would carry 5 VDC power as well, in many cases eliminating the requirement that peripheral devices include their own power supplies. In fact, rechargeable devices would be able to use USB to restore their internal batteries to full power.
Devices would be able to draw up to 5 unit loads from a USB port, with a unit load in USB 2.0 being defined as 100mA. “
Devices would initially function at low
Ruggedizing USB for Ground Loops
It is very useful to be able to connect a remote device via USB. But the greater the distance between the connected devices, the more likely it becomes that the communicating devices will be getting their power from different building ground references. When they do, the USB cable’s ground wire can create a ground loop path.
Figure 1. Ground voltage differences -
Your computers and connected devices should be protected with USB isolators. Ordinary surge suppression tries to limit spikes between the signal and ground line. But if the ground line rises, as it does in ground loop situations, then surge suppression will not stop it. Isolation allows the lines to float while keeping the local side at the proper ground and signal level. An isolator controls electrical flows while simultaneously ensuring that your data stream is not interrupted. You can deploy dedicated inline USB isolators as well as USB devices that perform additional functions while including isolation, like isolated hubs or serial converters. (See Figure 2.)
Figure 2. USB inline isolator with
Isolation works by changing the nature of both the data signal and the 5 VDC power carried by the USB cable. The data enters the isolator as an electrical signal, is converted to either
Figure 3. How optical isolation works.
The isolator protects against surges and ESD on the power line by briefly transforming the 5 VDC USB power to AC, through an isolation transformer, then back to DC again.
Ruggedizing USB Connectors
USB cables have some nice features. For example, the specification deliberately made it very hard to attach a USB connector incorrectly. The host devices that supply power use a Type A connector and the devices that receive power use a Type B connector. They cannot be reversed. Users cannot accidentally connect two USB power supplies and create a risk for fire or damage to
The specification also required that the power connections in the Type A connector should be on the
The connectors themselves were designed for easy insertion and extraction. The connectors would hold the cables in place with no need for thumbscrews or clips. That has proven to be quite handy in office environments, where people are often making connections at awkward angles, like under desks or behind various kinds of office equipment. It was a wonderful cabling solution for home and office computing.
But, the same easy connection and extraction that makes USB useful in an office environment can be a problem when you take USB off the desktop and out of the office. Heavy vibration in industrial applications can cause USB cables to work themselves loose. Any accidental tugging on the USB cable will produce the same result. And because USB carries power, a loose USB cable can cause arcing – a serious problem in environments that pose a risk for fire or explosion.
Manufacturers have started addressing the problem. One solution involves connecting USB cables and ports with thumbscrews. But, that requires the use of proprietary cables that are not likely to be available at your neighborhood office supply store. Additionally, the thumbscrews eliminate one of USB’s most useful features: the ease with which USB cables can be connected and disconnected.
A more elegant solution employs “high-retention” USB ports. They will work with any cable and they retain USB’s quick connect/disconnect feature. The difference is that it requires more force to dislodge them. The high-retention ports used by B+B SmartWorx, for example, hold a cable securely enough that it requires 15 Newtons of force (3.2 lbs) to remove one – so securely, in fact, that they will meet UL requirements for Class 1 Division 2 Hazardous Locations. (See Figure 4.)
Figure 4. High-retention USB ports grip standard USB cables tight.
Ruggedizing for EMI and ESD
Standard USB cables were intended for office environments, so they are not hardened against significant electrostatic discharges (ESD) or electromagnetic interference (EMI). ESD and EMI were not expected to be serious issues. But, as USB moves into places like crowded control panels, factory floors and emergency vehicles crammed with electronics of every description, EMI and ESD become a daily risk. In these tougher
More reliable USB cable is available in several versions. Options range from
Rugged Range Extension
USB has a range limitation of five meters. This is quite serviceable in office environments but less useful elsewhere. You can extend the range up to 30 meters using USB hubs, which also give you an opportunity to add additional ports. (USB allows a single computer to control as many as 127 different devices.) USB hubs designed for home/office are often rather flimsy, but you can install industrial-grade,
Consumer-grade USB devices are not built for hard use. When you move off the desktop, you will want to deploy better equipment and you may need to secure it firmly in place. When shopping for industrial grade hubs, isolators, USB-
USB failures can be expensive in ways that are not as immediately obvious as a burned out circuit board or connector. Equipment repair has associated labor costs, as does reprogramming.
“It is the price of convenience,” says Fahrion. “USB is too useful to ignore. But, whenever you take it off the desktop you’re going to have to employ a few workarounds. USB appears in every industry these days and there is a good chance that it was never designed for yours.
” USB is ubiquitous, it is useful and it is here to stay. But, it is not inherently rugged or reliable. It is up to you to make it that way.