Simple Design Tricks to Ensure Dependable USB Operation

Simple Design Tricks to Ensure Dependable USB Operation

USB is a critical connectivity path for AV system operation today, but the USB system, as we know it, wasn’t originally envisioned as a technology for a systems integrator’s toolbox. Universal Serial Bus (USB) was the result of an effort to standardize both the plug-and-play capability and the connectivity of computer peripherals.

The Universal Serial Bus system was developed about two decades ago by team consisting of Intel’s Ajay Bhatt along with Bala Cadambi, Shelagh Callahan, Shaun Knoll and Jeff Morriss. Today more than three billion USB ports are sold each year worldwide. It’s hard to imagine how many USB ports are in use in legacy equipment sold since the system really hit the market in 1998. In addition to the obvious computer world, USB is found on projectors and flat panel displays, phones, cameras, tablets, watches, e-readers and so much more. It’s become the very foundation of interactivity in electronics. Understanding USB technology isn’t optional anymore. To design, install or operate dependable systems we have to possess a good understanding of USB operation.

USB operates within a rigid framework of relationships and time windows. There are seven layers, or tiers, of USB connectivity. This is typically illustrated as a pyramid, with the top of the pyramid being the system host and the bottom consisting of all the devices ultimately connected to the system. In between are 5 tiers that are reserved exclusively for hubs. Hubs are very special devices.

A USB hub is a device that multiplies the number of ports so that there are more available to connect devices to the host. USB hubs are often built into equipment such as computers, keyboards, monitors, speakers, and especially into interactive white boards. A USB network starts with a USB host that’s inside of the computer or other controlling device. This host rarely is connected directly to the outside world. Instead, most devices that have more than one USB port typically split the output of the host via the first of a series of hubs. USB hubs are connected to upstream USB ports, which themselves may stem from USB hubs. The whole pyramid can extend a USB network to a maximum of 127 ports.

A hub has two jobs to do. First, it is a wiring aggregator. It allows, depending on its design, four or more downstream USB devices to be connected to a single upstream USB port. Second, the hub is a traffic controller. It “speaks” to all those downstream devices and then relays that message to the upstream host. To clarify, when you plug a thumb drive into a hub for example, the hub handles all communication with the thumb drive and all communication with the computer only allowing the data to flow through when very specific technical conditions are met. It goes without saying that both the hub and the host accomplish this signal negotiation in slices of time too small for us to notice.

Hubs come in one of two versions. They can be powered from the upstream source (bus powered), or they can have their own dedicated power supply. The rules of USB operation state that the host in current USB devices must be able to deliver a minimum of 500mA to each USB port. So if a hub is bus powered, then the downstream devices can only rely on a getting that much power to share between them. A four port bus powered hub must divvy up its 500mA total between its 4 downstream ports. The arithmetic makes it clear that it will be a problem if we plug two devices into the hub and they both need 300mA to operate properly. The hub also needs to power its own internal microprocessor chips from this limited power source, further complicating the installation.

Hubs are also found in USB extenders. When you install a 10 meter USB “active extender” cable, you are really adding a hub that has a single downstream port. Most of the time these extension cables are bus powered because we want a clean and easy installation without wall warts or bricks plugging into AC outlets. The same rules applies to these extenders; they can only count on 500mA from their host and they must power their own internal from that power supply. While there’s typically no problem in delivering the full 500mA to the far end (because host supplies often have a “little something extra” built into their delivery system), counting on a system to perform beyond its design parameters is not without risk. In the words of Steve Jobs, “Design in not just what it looks like and feels like. Design is how it works.”

A very good practice is to ensure that your installation never has more than two bus powered hubs or extenders connected in a row. At a minimum, every third hub inclusion should be self-powered. Considering a typical installation, the host is in the laptop computer. If there is more than one USB port on that computer the chances are very, very good that there is an internal hub. Most of the time we’ve occupied two tiers before we’ve even plugged in one cable. If the system is so simple that it is nothing more than an extender connecting an interactive white board, we might assume there is no need for a powered hub. But by now I’m sure you can see where we’re going with this. The extender is the third tier, and the passive hub in the IWB is the fourth. We’ve just violated our best practice!

Specifying powered hubs is good design practice. If you ensure that you have a powered USB 3.0 hub at every third tier, your systems will be dependable and robust.

By Joseph Cornwall, CTS-D
Technology Evangelist, Legrand N.A.

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