Isolated USB hubs? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: April 04, 2018, 06:10:09 pm
content: I am thinking about building myself an isolated USB Full Speed hub, so maybe I can have my debug probes attached to a live target without risking frying up my computer. How necessary is that, or is it a better idea to use an air gap isolated laptop (running on only batteries and Wi-Fi, no power or Ethernet cable attached) instead? According to Taobao it cost me less to build my own isolated USB Full-Speed Hub than buying one.Questions:1. Is it adequate to build using only USB 1.1 speeds?2. How do I power the downstream? Separate double isolated two-prong wall wart or internal power isolator?3. Is there any points I should be aware of when designing the PCB?

author: Benta
author_rank: Super Contributor
timestamp: April 04, 2018, 07:24:39 pm
content: What do you mean by "live target"?If at all possible, that's what you need to get isolated or potential-free. Floating your laptop (or other equipment) exposes you to being electrocuted from the "live target" voltages.

author: technix
author_rank: Super Contributor
timestamp: April 04, 2018, 08:35:44 pm
content: Quote from: Benta on April 04, 2018, 07:24:39 pmLive target means that target board being programmed or debugged have anything higher than 12V and capable of more than 6W sustained power running in it, regardless of isolation.One of my projects will involve a 24V to 5V 50A software controlled non-isolated DC-DC power converter, which is considered live target. Another project involves relay switching mains with a PoE-powered module, and since even without plugging the mains in, the PoE still supplies 48V at up to 30W, despite the isolation the board is still a live target.

author: Brutte
author_rank: Frequent Contributor
timestamp: April 04, 2018, 09:07:19 pm
content: USB full speed is cheap to isolate - there are USB isolators available on eBay (12$ delivered). The problem starts from USB high speed up.

author: e100
author_rank: Frequent Contributor
timestamp: April 05, 2018, 03:16:38 am
content: Quote from: Brutte on April 04, 2018, 09:07:19 pmHigh speed isolators do exist and they are much more expensive than the low/full speed isolators.I've seen high speed USB interfaces on devices that you wouldn't think would need a high speed interface, and these don't work with low/full speed isolators.If you are designing your own high speed isolator then you'll need test gear that can do 480 Mb/s.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 05, 2018, 04:04:53 am
content: Raspberry Pi + USBIP.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 05, 2018, 06:22:44 am
content: I recall the XDS100 (has been a very long time since I used one) is available in an isolated version. Even the cut down version found on the C2000 Launchpad is isolated.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 05, 2018, 06:26:42 am
content: 1. Depends what you want to do. You're not going to be using a logic analyzer at any speed with USB 1.1.2. The hub does, from it's external power supply.3. Don't design the PCB, just buy one. Buying all the parts and assembling it is more expensive.B&B Electronics UHR304 Isolated USB Hub teardown.I have this one, and it works great protecting the precious USB ports on my PC.Note that this hub does not float the downstream ports.Which would be dangerous anyway since USB plugs have exposed metal. Chassis is still connected to earth.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 05, 2018, 06:45:35 am
content: Quote from: blueskull on April 05, 2018, 06:36:45 amWell, there is your problem. Lack of copper earthing braids?

author: technix
author_rank: Super Contributor
timestamp: April 05, 2018, 07:19:24 am
content: Quote from: Jeroen3 on April 05, 2018, 06:26:42 am1. For me all I would attach would be JTAG debug probes and USB Serial adapters.2. So a wall wart would work...3. I actually want to float the downstream ports...As of the air gapped laptop, I intend to VNC, RDP or SSH into it for debug sessions. Code is written and built on my workstation, checked into version control on my server, checked out on the laptop, debug session is launched there (using a GDB server) and controlled on my workstation. There is no reliability of USBIP involved.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 06, 2018, 02:20:06 am
content: Quote from: blueskull on April 05, 2018, 06:36:45 amLooking at the C2000 Launchpad, it looks pretty easy to remove the isolators, cut the board in half, and then replace the isolators with some rated for a higher voltage - likely short range fiber transmitter/receiver pairs. Or just build some fiber isolation adapters without modifying the XDS. There should be no need to isolate something as complex as USB.I wonder if another solution could be a USB over RJ45 kit and an Ethernet lightning arrestor, if isolation is not really needed during normal operation.