I would like to buy a USB switch and hub, or a combination hub/switch to connect several USB peripherals to two computers. According to this answer on a related question, many USB devices like this are cheaply designed and can feed power from host to host or from device to host. I would like to use a multimeter or similar device to ensure that my hubs and switches will not damage my devices. What is the best way to do this?
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\$\begingroup\$ Plug it into one of the computers and measure if there's a voltage on the line that would go to the other computer. \$\endgroup\$– HearthCommented Dec 26, 2020 at 16:25
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\$\begingroup\$ There is a test method specified by USB-if I would look at the compliance testing requirement at usb.org for example there is a tool that accepts scope traces to validate device inrush usb.org/compliancetools#anchor_electricaltools \$\endgroup\$– crasicCommented Dec 26, 2020 at 16:33
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The test for backfeed is fairly simple. You need to make a dummy USB port with VBUS, D+ and D- loaded with 15 k resistors. When you plug your hub-switch combination into this "fake"/test port, voltages on any of the pins (VBUS, D+, D- ) should not exceed 400 mV. See the Back Voltage test description here. For schematics of the test fixture, see this answer, USB IF specification for back powering Hubs .
For general scope of USB 2.0 electrical tests for USB-IF compliance, see this document.
answered Dec 29, 2020 at 4:57
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\$\begingroup\$ Why could I not just check the voltage on the VBUS, D+, and D- pins relative to ground on the hub itself? \$\endgroup\$ Commented Dec 29, 2020 at 21:58
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\$\begingroup\$ @Danny, you surely can, but the result might be misleading. The pins might have small leakage (as any silicon has), so there could be some voltage readings. Having nominal pull downs and having under 400 mV signal ensures that no false signaling or damage to PC port will occur. \$\endgroup\$ Commented Dec 29, 2020 at 23:02