I'm experimenting with a multiple hub "tree" with an array of serial
devices at the leafs and have arrived at some questions. The first regards
errors of the form "usb-uhci.c: interrupt, status 3, frame# 2038". These
errors show up regardless of the configuration I use, but the frequency
increases if I use non-powered or bus-powered hubs. In fact, they increase
to the point that three hubs deep with 5m interconnects, the serial
converter will usually fail to be recognized or perform poorly. I presume
that this error represents signal degradation, since if I use self-powered
hubs, then I see fewer errors or no failure of the device. Now here is the
confusion: Why does signal degradation decrease if I use self-powered hubs
as opposed to bus-powered hubs; they are still powered right? Or does a
non-powered or bus-powered hub enter some kind of passive hub mode? To be
clear, I have the following arrangement:
[computer]---5m---[self-powered hub]---5m---[self-powered
hub]---5m---[self-powered hub]---[usb-serial]
This setup works fine, though I do see some "interrupt" errors. If I
now pull the power supplies connectors from the self-powered hubs (or any
two for that matter) then the frequency of the "interrupt" errors increases
and often the usb-serial converter fails even though the hubs are still
*powered* by the USB bus. Is this strange?
As a side question, consider a tree of usb hubs at or near the
theoretical maximum number of devices, all of which are self-powered.
Further consider that the sum of the current draw of all these hubs vastly
exceeds the capability of the computer's USB port (seems likely). If those
hubs are not on UPS but the computer is and there is a power outage
instantaneously forcing all the hubs to bus-powered mode, might this damage
the USB port on the computer?
I should mention that the usb-serial adaptors I am using are all
pl2303 based devices, and I am running RedHat 7.3 (2.4.18-4) on a PII-266.
-poul
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