USB Hub Contenders: Hub X

We have disguised the brand names of these four hub suppliers, referring to them with single letters: “X”, “T”, “L” and “S”. Each of these hubs were purchased from reputable online vendors and were advertised for use in industrial environments. All except Hub T claimed to have ESD protection up to ±15kV; Hub T did not specify an ESD rating. The prices for the hubs varied widely, too. The following table shows the range of prices found for the same brand and model of each hub.

Hub X    $120-149
Hub T    $99-119
Hub S    $40-79
Hub L    $53-69

First up is Hubs X and T. You’re not seeing double. Hubs X and T are exactly the same under their metal covers. Both hubs are branded by European companies, but the hardware is actually made by a Taiwanese ODM. White labeling is common for commodity products like USB 4-port hubs. Unfortunately it means that repeat purchases of the same brand and model might result in different hardware since the vendor may switch to other white label hardware any time. If you’re trying to scale out production systems, you need to know you can get the same hardware every time, but this can be difficult when relying on white label vendors. We’ll drop Hub “T” and just look at Hub “X”.

Hub X

Hub X, in its various incarnations, claims ±15kV ESD protection or just “industrial”. This hub has no external ESD protection, and it uses the ViaLabs VL811 chipset. Data sheets for the VL811 aren’t available. The VL812 claims ESD immunity to ±2kV HBM. The HBM specification uses far lower current than the IEC-61000-4-2 standard, so a 2kV HBM is 90 times lower power than the IEC standard. Assuming the VL811 is similar, the 2kV HBM ESD specification of the VL812 is likely not useful in production environments, so the chipset should have external ESD and transient voltage suppression added.

Unfortunately, Hub X does not add any extra ESD or transient voltage protection to USB signals, meaning common ESD strikes when cables are connected can affect the sensitive hub controller. The USB connector is designed to make connections in a specific order; shield, ground and power (Vbus) and then data lines. Careful design handling of the shield and ground signals can add a reasonable level of protection against this common strike vector. However, Hub X’s design directly ties USB shield and ground at the USB connectors. As such, any ESD event from connecting a cable would likely be directly injected into the ground plane of the hub controller. Such events could force transient currents of 30-100 amps across the ground plane, through the hub controller IC and into the supply planes and data lines. These events could also raise the ground plane which are connected to the host computer and other devices, causing cascading failures across the bus. Inappropriately handled ESD events like this could cause resets or irreparable damage to the hub or even a connected host computer. Such damage would dramatically raise the ”cost” of this cheap hub.

Hub X does have several ferrite beads (SMT and through-hole) on the external power inputs. Their placement and sizing would make them useful only for noise suppression, not ESD or surge currents. Also, bi-directional transients would completely bypass the ferrite beads through the shield connection to ground.

Hub X has footprints for common-mode chokes on the USB differential data lines, but these were not installed. Usually such common-mode chokes are used for noise suppression and EMI/EMC compliance, but they can also be useful for mitigating some energy from ESD currents. Adding these chokes would be “better than nothing”, but hub X opts for nothing.

The final ESD related design flaw in Hub X: LEDs. Simple indicator LEDs are useful and seem innocuous from an ESD perspective. However, LEDs are notoriously sensitive to ESD, and Hub X has 5 through-hole LEDs pushed through its metal housing. These protrusions are essentially ESD antennas. Many times, an ESD strike to an LED will simply destroy the LED, but in other cases, the currents will travel into the ground planes, power planes, and any drive circuitry. In Hub X’s design, these LEDs are driven by the hub controller without even a current limiting resistor in the series. An ESD strike to one of its LEDs would likely destroy the hub controller.

While evaluating hub X, we had 2 of the 4 downstream ports stop working just from plugging in standard devices. This occurred in normal lab environment, without applying any kind of stress testing. We suspect ESD damage to the hub controller.