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Making USB Reliable in Rugged Applications



Mike Fahrion from B&B Electronics provides some tips on using Industrial USB in the field. He explains the types of connections, mounting options and how to choose the right product for your application.

Transcript for USB Tech Tips: Making USB Reliable in Rugged Applications Video

You can hardly buy a piece of electronics gear today that doesn't come with a USB port on it - cameras, phones toys, even my daughter's Legos come with a USB port.  So over the years, despite a bit of a rocky start, USB has become quite a success for desktop applications.
 
My name is Mike Fahrion and I've been involved with USB since its pre-release days in the mid-1990s.
 
In that time, I've come to learn a lot about how to take USB into applications off the desktop where ruggedness and reliability are really critical. In the next few minutes, I'd like to share a few the inherent weaknesses with USB with you and, more importantly, talk about the solutions.
 
So, if you're at all interested in using a USB port for any kind of communications of of the desktop, hang with me for the next few minutes.
 
So, problem number one is electromagnetic immunity, or EMI. So, USB, once you take it off the desktop and into these more rugged applications you typically run into much higher levels at EMI electrical interference.
 
That happens for two reasons. One is, you're around more heavy industrial equipment, you know, high voltages, switching high currents, things like that that causes havoc with interference. 
 
And, second, as you tend to pack a lot electronics into a tight area, much tighter than what you're going to see under your desk in your home office --  you've got a lot of equipment packed into a medical cart, packed into the trunk of a squad car, packed into some rack or cabinet on the factory floor so that proximity causes a a much higher degree of electromagnetic interference. The result of that, typically with USB is data errors.
 
Worst case, you can actually blow a port. Those data errors with USB are a bit problematic because USB isn't quite as robust against data errors as some other technologies. It's not uncommon to see an application actually hang because of some data errors that happened during a surge of some sort or worse case the PC might actually blue screen.
 
In any case, now yours application is down until somebody comes out and resets the devices. So that's a situation you really need to avoid in rugged and reliable applications where reliability is critical. So the solution there is to design the products to a higher standard.
 
So instead of designing to the consumer electronics standard that most electronics is built for, we design to a heavy industrial standard which means it's designed to withstand the ESD events, the fast transient surge events, higher radiated immunity, higher conducted immunity, all of the things that can wreak havoc with the reliability of electronics and causing errors in data streams.
 
So, here at B&B, our USB hubs, our USB isolators and USB to serial converters are typically designed to those heavy industrial EMC standards to help prevent those problems in the field.
 
Problem number two with using USB in those environments is isolation. So any time you connect two wires together over a little bit of distance or between two different power systems you create an opportunity for a ground loop. Now that's not a big deal at home when you're connecting your PC to your desktop printer -- no real risk of creating a significant ground loop there. 
 
But what if you're connecting a PLC in one cabinet to a drive in another cabinet in an industrial situation?
 
Or you've got a medical cart that gets powered by one power supply and plugs into a USB device into a PC that's plugged into a different power supply. All kinds of situations there where you can create a ground loop. So a ground loop is a real problem at two levels.
 
First, it can cause... best case scenario I causes data errors and we just talked about what a data error can do to USB. Worst case scenario, and this happens very frequently, it's a little larger current flowing through that ground loop that you've created and something in that connection has to act like a fuse.
 
It's mother nature, you're not going to overcome it. So something in that equipment, in that chain, and it's usually something pretty expensive, is going to act like a fuse in order to break that current flow. So what's the solution is to add isolation.
 
So what isolation effectively is... is we're going to convert the data signals, either through optics or through magnetics,  convert them through that media then back to electrical signal again on the other side.
 
So what that does is it totally breaks the current flow from one side to the other so now there's no loop anymore yet lets your data pass unaffected. So now when a ground event happens, you know, it's invisible to the system because we don't let any current flow through there. So not only do we prevent damage in the equipment but we actually eliminate the issue with data errors as well.
 
So here at B&B we're pretty big on isolation. We can solve that problem a number ways in USB. We have in-line USB isolators. You have USB on one side, USB on the other and you just stick it right in line with your product. We've got the same thing in a smaller form factor for certain applications.
 
If you're connecting to a serial device which might have a really long cable on it on one side, we have isolated USB to 232 and isolated USB to 485 converters to be sure that you break the ground loop that way. We even have isolated USB hubs. If you're in a situation where you're adding additional USB peripherals and you need to isolate it from the upstream side, you can do with a nice isolated USB hub.
 
Problem number three is quite simply - connectors. So USB connectors, they're adequate for the consumer world but frankly they leave a little to be desired.. they're a little disappointing once you get them off the desktop. A lot of insertions and they start to get loose and even in the best-case it doesn't have a lot of retention force there. So you have to worry about if you're going to install USB in that situation where you've got dangling cables, maybe there's some vibration going on, any issues like that. The connector just isn't robust enough for that situation.
 
So at B&B we've solved that problem by using up what we call a high retention connector.
 
And you can always identify our high retention connectors by this orange color. And really, these use a standard USB cables. So that's a nice feature. You don't have to go out and buy a special USB cable because you're guaranteed not to have that on the job site when you need it. But they they just offer a much higher retention force, you can hear that thing click in there. That gives you a really strong connection that's going to last. It doesn't wear out in just the first few hundred insertion cycles like you see in a lot of consumer grade USB.
 
The other thing is that this connection is even strong enough that Underwriters Laboratories has approved this connector for use in hazardous environments where explosive gases can be present. It's important to those guys to not have a cable that can fall out because that can cause a spark and that causes bad things to happen. So, even Underwriters Laboratories agrees that this connector is strong enough to solve the problem in industrial environments.
 
Ok, on to problem number four. The real problem here is just packaging and specifications. The reality is, most USB equipment was destined to be installed on your desktop. You know, it's going to be laying behind you desk, it's going to be under the couch, it's going to be in a nice comfortable cabinet someplace. It wasn't designed for these more rugged and reliable applications.
 
So you end up buying consumer electronics USB products, you know that there's no way to mount them, the temperature specs are usually -- you need to be in a temperature-controlled environment, They often require AC power. Well in the industrial world or other real world kind of machine to machine applications you don't always have an AC outlet available. The temperature isn't always between 0 and 50 degrees C or even 0 to 40 degrees C.
 
And you'd like to be able to mount the product. You really don't want to be using duct tape and velcro all the time out in the field. So at B&B we've kind of taken this on and said we're always going to provide good mounting options for products. So you've got things like mounting ears so you can panel mount a product. We've got products with a DIN rail mount option so you can hang it on an industrial DIN rail. Or if  you don't have DIN rail and you want to panel mount it we include these little brackets so you can panel mount it up side up or you can mount it upside down or you can panel mount it to the front of a panel or to the back of a panel. Just lots of mounting options. 
 
And finally, we don't rely on the fact that you're going to have 120 volts or 240 volts AC available all the time. So we typically put on an accessory terminal block that allows you to bring in 10 to 30 or 10 to 40 volts DC as well. So you don't have to find an AC source to power the product if it requires power.
 
So that's it. Those are the top issues that I've run across with USB in terms of how to make it rugged and reliable in those mission-critical type applications or even if not mission critical, critical enough that you don't want a phone call and have to go out and service this thing out in the field.
 
If you need any more information about USB, we've got lots of great articles and product information over at the B&B website at www.bb-elec.com
 
Thank You.