Electronic – How reliable is a daisy-chained USB bus? What are the risks and how can they be minimised



USB has been around for quite a long time now, used extensively even in the automotive, marine and space industries to some extent. However certain communication ways are more reliable than others in terms of:

  • Possibility of hot redundancy

  • Protection of the connected equipment against failures in other connections (surges, short circuits, misbehaviours [data, voltage levels, timing…]…) or in the hub/switch itself

  • Large mean time before failure ("freeze" or hardware failure)

Usually, the error rate is a bit less important as detection and correction (which may require retransmission) is handled by the protocol library/stack.


I would like to know how reliable is a system based on a USB hub (say, 16 ports, be it daisy chained or not). What can commonly go wrong, and what could be done (protections/topologies…) to prevent single point failures from spreading (ideally I would just swap the faulty equipment and it would be ready to go).

Specific problem

For a specific piece of equipment at work I need to interface 16 devices to both a primary PC and a redundant PC (taking over if the other one crashes). 75% of the devices have both Ethernet and USB and 25% are only USB, and I'm wondering if I should simplify everything (which may increase reliability as well) by using USB alone or have both to maximise reliability. For USB, switching between the 2 computers would be done using a manual switch.

To illustrate, the hub I'm considering is this one, handling < 350W surges but doesn't seem isolated; I have the hunch they're overusing the term "industrial" so I may switch it for several daisy chained 7 ports. The connected equipment is a bunch of industrial PID temperature controllers, a UPS and a precision thermometer. However this question is more of a general one.

Best Answer

You are correct in that surges are a concern for cascade failures; it is also true that USB is not galvanically isolated by default, and in fact is a pain in the rump to isolate due to its half-duplex, bidirectional, differential-but-not-always (blast the SE0!) nature. Thankfully, the good folks at Analog Devices stepped up and put together the ADuM4160 to take care of all of the truly hard parts of USB isolation -- while it only supports low or full speed operation, and cannot pass low/full speed negotiation signals, meaning it can't be built into a general purpose isolated USB "lump" without help, it is still the closest thing to a generic USB isolator available.

Assuming that a reliable 5V supply to the hubs is available, and preferably one 5V supply per hub (not hard!), I would split the hubs into 4 ports each and isolate the upstream port for each hub -- pick your favorite hub chipset here. Along with good per-port surge suppression, for which I'd specify a pair of Bourns TBU devices in series with D+/D- and a shunt low-capacitance, high-energy suppressor in addition to a MOV and TVSS clamp + current limiter or PPTC for Vbus and the obligatory TVSS/clamp network for D+/D-, this should limit the propagation of surges through the USB network.