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USB Isolation: Things to Consider

10/22/2018

Isolated USB protects your computer’s USB port from dangerous ground loops and provides data integrity in electrically noisy environments; such as near a variable frequency motor drives, welding or other large loads. However, this added protection and reliability do not come without a price. Before deciding to use USB isolation, you need to consider your bandwidth, downstream power and cascade requirements. 

Bandwidth

The USB standard has undergone several revisions – each impacting bandwidth, speed, data rate and more. The USB 2.0 standard calls out three speeds. The first two speeds are carryovers from the earlier USB 1.1 standard: Low Speed and Full Speed. When USB 2.0 came out, High Speed was introduced. USB 2.0 is also backwards compatible with the lower USB 1.1 speeds. The naming convention can cause a bit of confusion because “Full Speed” is lower than “High Speed.”

 Lastly, USB 3.0 “SuperSpeed” adds even higher data rates, more downstream device power, plus smart charging/powering. 

Table 1 below shows actual data rates at various USB standards.

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Isolation Bandwidth Limitation Factor 

Adding isolation limits bandwidth to “Full Speed” or 12 Mbps. While this is still technically USB 2.0, the host controller will not pass data at “High Speed” or 480 Mbps. It will automatically step down to 12 Mbps. Luckily, most industrial applications will work well at 12 Mbps. If you require 480 Mbps “High Speed,” you should not use isolation or, perhaps, you might consider USB 3.0 SuperSpeed. B+B SmartWorx offers USB 2.0 Hubs and USB 3.0 SuperSpeed Hubs, plus isolated and non-isolated options as shown in Table 2 below.

 
 

Important to note is that, while each version of USB is backwards compatible with its predecessors, it cannot make earlier version devices go any faster. For example, a USB 3.0 port can communicate with a USB 1.1 or 2.0 device, but only at the USB 1.1 or 2.0 speed inherent to the device. 

The reverse is also true. A USB 3.0 device attached to a USB 2.0 port can only operate at USB 2.0 speeds. To get full USB 3.0 speed, both the host USB port and the connected device must be USB 3.0 capable. 

B+B SmartWorx also has a selection of one or two port USB isolators. The UH4xx product series are fixed at either 12 Mbps or 1.5 Mbps speeds. This is an important distinction because “Low Speed” isolators will not work with “Full Speed” devices and vice versa. 

The UHRxx isolators also contain a USB hub chip. The addition of the hub chip makes them “smart” in that they can self-select between 1.5 Mbps “Low Speed” and 12 Mbps “Full Speed.” None of these isolators are capable of operating at “High Speed” 480 Mbps. The following Table 3 shows bandwidth capabilities of B+B SmartWorx USB port isolators.

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Power 

Another aspect to consider is the power requirement. B+B SmartWorx isolated USB hubs, both four and seven port versions, require the use of an external power supply. As a powered hub, they provide 500 mA through each port to power downstream devices. 

Models UH201 and UH40x series isolators do not have an external power option and are limited to 100 mA downstream power. 

Model BB-UHR204 has the added option of operating in a USB bus-powered mode. In this mode, downstream power is limited to 100 mA from each port. 

Models UHR401 and UHR402 must be powered from an external source and provide 500 mA downstream power from each port.

 
 

Cascading 

Finally, USB isolation limits the number of hubs that can be cascaded. This is typically rare-case use, but it is worth discussion. 

The USB 2.0 standard allows up to five hubs to be cascaded. This includes the USB controller hub built into the PC. If you are using an isolated USB hub, the isolation chip introduces a propagation delay equivalent to one USB hub. Therefore, the isolated hub counts as two hubs in the cascade. So, you can only hook together two isolated hubs on a single USB host controller. 

Under USB 3.0 isolation requirements, if a USB 3.0 hub is connected to a USB 2.0 host, downstream ports will not accept USB 3.0 devices. Also, cascading multiple units is not supported.

B+B SmartWorx - Free Technical Support The key to selecting the correct USB hub or isolation product is to understand your application. The information provided above should make this easier for you. If you have additional questions, please feel free to contact B+B SmartWorx Technical Support staff at 1 (800) 346-3119 or via online chat. They will guide you in the correct direction and product selection for your application.

 
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