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List:       linux-usb-users
Subject:    Re: [Linux-usb-users] Problems with FTDI Serial Devices
From:       Alan Stern <stern () rowland ! harvard ! edu>
Date:       2005-08-31 14:39:08
Message-ID: Pine.LNX.4.44L0.0508311037050.4937-100000 () iolanthe ! rowland ! org
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On Wed, 31 Aug 2005, Andy Dalton wrote:

> Greetings,
> 
> I'm attempting to connect five FTDI serial devices (FT232BM) to my
> Linux system (2.6.13) via a 7-port USB hub.  I am able to connect and
> communicate with up to three devices successfully (one device at a
> time).
> 
> Adding additional devices cause problems.  In the scenario I've
> captured here, adding the fourth device causes intermittent
> communications failures.  When attempting to add the fifth device, the
> device does not get recognized.  Other times, I've gotten similar, but
> different results with more than three devices.
> 
> The hub is connected to the system via a 15' USB cable, and each
> device is connected to the hub via 15' USB cables.
> 
> Any ideas you might have would be appreciated.  Are the cables too
> long?  Am I expecting too much by connected so many of these devices
> to a single hub?  Do I need a better USB controller?  Do I need a
> better USB hub?  Is there some practical limit to the number of
> devices I should expect to be able to attach at a given time?

It's possible that the cable length could cause problems; you're running 
up against the limits of the USB spec there.

More likely your problem comes from plugging full-speed devices into a hub
running at high speed.  Try "rmmod ehci-hcd" so that everything is forced
to run at full speed, and see if that helps.

Alan Stern



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