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USB Isolation – Things to Consider

Isolation protects your computer’s USB port from dangerous ground loops and provides for data integrity in electrically noisy environments; such as near a variable frequency motor drive, 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 2.0 standard calls out three speeds. The first two speeds are actually 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. As you can see, the naming convention can cause a bit of confusion because “Full Speed” is lower than “High Speed.” Be careful when spec’ing out your equipment.
 
Name Standard Speed
Low Speed USB 1.1 1.5 Mbps
Full Speed USB 1.1 12 Mbps
High Speed USB 2.0 480 Mbps

 
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 ratchet down to 12 Mbps. Therefore, if you require 480 Mbps “High Speed,” you should not use isolation. Luckily, most industrial applications will work fine at 12 Mbps. Our USB Hubs come in both isolated and non-isolated versions.

Model Isolation Ports Low Speed USB 1.1
(1.5 Mbps)
Full Speed USB 1.1
(12 Mbps)
High Speed USB 2.0
(480 Mbps)
UHR204 None 4 YES YES YES
UHR207 None 7 YES YES YES
UHR304 4 kV 4 YES YES NO
UHR307 4 kV 7 YES YES NO


We also have a number of one or two port USB isolators. The UH4xx series products are fixed at a particular speed, either 12 Mbps or 1.5 Mbps. This is an important distinction because “Low Speed” isolators will not work with “Full Speed” devices and vice versa. The UHRxx series isolators also contain a USB hub chip. The addition of the hub chip makes them “smart” in the sense that they can self-select between 1.5 Mbps “Low Speed” and 12 Mbps “Full Speed.” None of the port  isolators are capable of operating at “High Speed” 480 Mbps. This reference chart shows the bandwidth capabilities of each of our port isolators.

Model Isolation Ports Low Speed USB 1.1
(1.5 Mbps)
Full Speed USB 1.1
(12 Mbps)
High Speed USB 2.0
(480 Mbps)
UH201 2 kV 1 NO YES NO
UH401 4 kV 1 NO YES NO
UH401-2KV 2 kV 1 NO YES NO
UH401SL 4 kV 1 YES NO NO
UH401SL-2KV 2 kV 1 YES NO NO
UHR401 4 kV 1 YES YES NO
UHR402 4 kV 2 YES YES NO


Power

Another thing to consider is the power requirement. Our isolated USB Hubs, both the 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. The non-isolated four port hub also has the option of operating in a USB bus powered mode. In this mode, downstream power is limited to 100 mA from each port. The UHR401 and UHR402 must be powered from an external source and provide 500 mA downstream power from each port. The UH201 and UH40x family of isolators do not have an external power option and are limited to 100 mA downstream power..

Model USB Port Power External
10 to 30 VDC
Downstream Power
Available
UHR204 YES YES 100 mA per port or 500 mA per port, external power
UHR207 N/A YES 500 mA per port
UHR304 N/A YES 500 mA per port
UHR307 N/A YES 500 mA per port
UH201 N/A NO 500 mA per port
UH401 YES NO 100 mA
UH401-2KV YES NO 100 mA
UH401SL YES NO 100 mA
UH401SL-2KV YES NO 100 mA
UHR401 NO YES 500 mA per port
URH402 NO YES 500 mA per port


Cascading

Finally, USB Isolation limits the number of hubs that can be cascaded. This is a rare use case, but it is worth mentioning. 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.
 
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 our support staff at 800-346-3119. They will guide you in the right direction.