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Incompatible Self-Powered Hub

morgaine

[This thread was split off from an earlier general one about Powered Hubs and has been given a more appropriate subject line.  Note that replies prefix "Re:" to the old subject line.]

 

 

The Pi's very poor power design results in some perfectly good self-powered hubs being incompatible with it.  For example, the cheap little 7-port self-powered Dynamode USB-H70-1A2.0 hub that Farnell advertises as a Pi accessory:

 

 

is not actually Pi-compatible at all, because in addition to powering its 7 host ports, it also powers the Pi board through its A-B upstream lead yet is incapable of supplying enough power to do that successfully (the Pi's Vcc drops to well below 4V).  As a result, when the micro-USB is powered down but the hub is powered up and connected, the Pi ends up rebooting continuously every couple of seconds, potentially leading to destroyed HDMI displays among other worries.

 

So, buying a self-powered hub is not necessarily the end to your Pi power problems.

 

Really the Pi's power design needs to be nuked from orbit and redesigned from scratch.  It's just plain bad in numerous ways.

 

Morgaine.

Parents

  • I'm not sure it's the RPi design that's wrong, more that USB hubs are not always very careful about where their power goes. For instance... I have a Logik LP4HUB10. But, it sends power through the hub-Pi cable (just like Morgaine's problem) and I had to follow instructions http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=8926 here that meant taking apart the hub, snipping a wire and putting it back together again.

    I guess the only thing the Pi could do is to prevent incoming power into the two USB ports... But then again why should it - power going through the wrong wire in a hub is a power drain, surely?

  • in reply to recantha

    Michael Horne wrote:

     

    I'm not sure it's the RPi design that's wrong, more that USB hubs are not always very careful about where their power goes. For instance... I have a Logik LP4HUB10. But, it sends power through the hub-Pi cable (just like Morgaine's problem) and I had to follow instructions http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=8926 here that meant taking apart the hub, snipping a wire and putting it back together again.

    I guess the only thing the Pi could do is to prevent incoming power into the two USB ports... But then again why should it - power going through the wrong wire in a hub is a power drain, surely?

    RasPi shouldn't have to do anything.  A well-designed powered hub should not be sending 5V current upstream.  My cheap Inland 4-port hub has a Schottky diode to make sure this doesn't happen.  A better solution is to use a hot swap controller to reduce voltage drop, but that adds cost.  Hubs should not directly connect 5V external power to the upstream line, but obviously some do.

  • in reply to johnbeetem

    John, I think what's most shocking is that I got my hub from PC World, of all places. You would've thought a high street chain like that would only sell things of the highest quality... Mind you, this is the company that popularised the Packard Bell make of PCs, so...

     

    Morgaine, I wasn't attempting to hijack the thread, just seeking to contribute to the discussion... I may have made the wrong assumption. I'm assuming that NO device (Pi, PC whatever) is supposed to be powered through a type A socket that is specifically there to provide data inputs?

  • in reply to recantha

    That's correct, but because self-powered hubs can usually also work in bus-powered mode and hence draw power from their host, it seems to be common for them to link their powered +5V directly upstream as well.  Unfortunately this is bad for the Pi because it starts her boot cycle, despite successful booting being 100% impossible because of the designed F1/F2 polyfuse values.

     

    Of course it's nothing new that two devices are not compatible, especially when one of them has a pretty oddball power design.  In this situation though, a device that isn't compatible with the Pi's idiosyncracies should not be marketed as an official Pi accessory.

Reply
  • in reply to recantha

    That's correct, but because self-powered hubs can usually also work in bus-powered mode and hence draw power from their host, it seems to be common for them to link their powered +5V directly upstream as well.  Unfortunately this is bad for the Pi because it starts her boot cycle, despite successful booting being 100% impossible because of the designed F1/F2 polyfuse values.

     

    Of course it's nothing new that two devices are not compatible, especially when one of them has a pretty oddball power design.  In this situation though, a device that isn't compatible with the Pi's idiosyncracies should not be marketed as an official Pi accessory.

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