I converted an old IBM model M to USB with a hub inside (I want only one
wire on my desktop). It turns out that the keyboard sucks a lot of
power. I can't use a thumb drive on this thing without plugging in the
wall wart that came with the hub.

Will a powered hub upstream pass the power downstream?

Previously Mark Healey <die (AT) spammer (DOT) die> wrote:
Not really. The hub will give you 500mA per port (typically fused,
so no overloading) and that is what you are getting out of a
mainboard port as well.

I assume what you get is blackout on the keyboard and hub if you
plug in your memory stick? If so, one thing you can do is increase
the fuse rating (typically a resettable fuse, otherwise the power
would not come back up) with the same model, but rated at 1A
instead of 500mA. This should still be within what the cable
can carry safely. Downside: This requires you to modify either
your mainboard or that additional hub and you need to find
a matching fuse. Usable fuses could, e.g., be Tyco RUE110 or
RXE110, Wickmann RLD06P120B, Bourns RLD06P120B or the like.

Arno

On 27 Apr 2007 16:00:56 GMT, Arno Wagner <me (AT) privacy (DOT) net> wrote:

AIUI compliant hubs (inboard/outboard/motherboard) will only supply 100mA per
port untilo the connected device "negotiates" for more, and then the 500mA
figure comes into play.

Also, on all the USB ports/hubs I have seen there are no fuses to limit the port
supplied current - only electronic current limiting.

See above

Previously rebel <me (AT) privacy (DOT) net> wrote:
Hmm. It is possible that the hub I looked into was a cheaper/older
model. It had 500mA polyfuses.

Ok, if there are no fuses but limiting circuits, then this approach
does only work in a modified form: Supply the +5V directly via
a fuse. Omiting the fuse is not a good idea, since a PC PSU
packs enough current reserves to explode wires if shortened.

Arno

"Arno Wagner" <me (AT) privacy (DOT) net> wrote

USB ports are not permitted to be fused as such (they cannot claim
compliance if they are). The spec requires overcurrent protection, but it
must be self resetting without user intervention.

500mA per port is only available from a self powered hub (i.e. one that gets
its power from a separate power supply). If the hub is bus powered (if the
hub supports such operation), then the output is limited to 100mA per port.

It's not quite as simple as upping the current limiting. The USB spec
requires any USB port to supply a guaranteed 100mA of current, regardless of
how it is powered. Additional current, up to 500mA, is theoretically only
available by negotiation with the host. The actual value of the current
limit is software controlled. In practice, however, the port current limit
for self powered hubs gets set to the full current when the port is
enumerated by the BIOS (when looking for bootable USB devices). It will
stay at this maximum value if the Windows host driver has no reason to
reduce it. The actual current that can be drawn is actually higher than the
advertised limit and the port must be capable of delivering much larger
transient currents.

The usual way of getting more current for a single device is to use two
ports.