Let's walk through the capabilities & limitations of USB devices
(especially hubs) and how that applies to your situation.
First, USB devices can draw power from the host (or upstream hub)
- freely up to 100 mA
- or after negotiation for additional power, up to 500 mA
to a maximum of 500 mA per port. This is described in general terms at
as well as at
as part of an intermediate level explanation of the USB protocols and at
which has a link to the USB standard which describes the capabilities
/ limitations in great detail.
I would be very careful at reviewing the power requirements of each
external device and separating the high power devices onto separate
USB ports (using more than one hub or cascading self-powered hubs if
necessary) to stay within these limits.
As a side note, there is a related "USB plus power" standard described at
which describes a compatible protocol with much higher power
capabilities. It may be that some of your devices are expecting this
higher power protocol. There are a couple vendors referenced at this
site such as
which describe several devices and suggesting uses in "Point of Sale"
applications. I have not reviewed the standard in detail - I would
certainly do so before attempting to replace a standard USB hub with
one of these devices.
The more "standard" USB hubs are devices such as
A seven port powered hub from "Dr. Bott" which indicates nominal
current limits of 500 mA, with overcurrent protection as high as 1250
mA (may be temperature related). I found a couple reviews of this
device (search using the product name) & they were both positive about
a four port device that appears from the picture to be bus powered,
but indicates it is self powered (by capability & by the power supply
listed near the bottom).
another seven port self-powered hub that indicates it provides 500 mA
per port. Very inexpensive, I believe its USB 1.x only (not USB 2).
another four port self-powered hub (with a bus powered hub) that is
USB 2.0 compatible. When I did a quick product search (using
Froogle), there are a couple thousand USB hubs out there. The products
referred to above all indicated that they fully met the 500 mA
capability per port (and were relatively easy to find).
For some additional background information related to power limits, see
which describes a four port USB interface device from Texas
Instruments. If you review the information, it indicates the "over
current" condition is detected in more than one way:
- by current sensing circuits
- by temperature of the core circuits
Apparently, what works well for one situation does not work well with
others. It may be that kind of situation that describes the reported
limits of the "Dr Bott's hub" (you can draw more power if the device
For additional information, try search phrases such as
maximum current USB
self-powered USB hub 500ma overload protection
and similar phrases.
If some part of the answer is unclear or you need additional
information on some aspect of this question, please make a
clarification request. I would be glad to add more to the answer as
Good luck with your work.