RocketMan on 7/4/2010 at 19:56
Hey all. I've got an iPAQ 210 PDA and I recently discovered that the stupid proprietary 24 pin connector that does nothing more than charge and sync (same as any usb cable) can also be used for a variety of other features, none of which were actually implemented by HP when the device was released. Among these there is support for stereo out / mic in via the cable as well as USB host capabilities. While one would normally need to purchase an expansion card to enable host capabilities, this unit has been capable of it all along and supports HID devices as well as mass storage devices. All you need to do is rip apart the sync cable and do a little surgery. To this end I've started making a 24x to USB famale cable. I have a concern though.
As I expected, upon disecting a USB extension cable which has the female end on it, I found the usual 4 wires plus what looks like a ground or shield wire (uninsulated). I "believe" this is attached to the chassis of the female plug though I could be wrong. The metal chassis of the 24x sync connector also had an uninsulated wire soldered to it and my intuition tells me I should solder the USB cable's ground wire to the 24x chassis. However when I look at the plans on the internet which demonstrate how to make this cable I see no evidence whatsoever that any attempt was made to deal with this wire. Rather it was snipped and forgotten. According to the author the cable works fine and I've made circuits in the past with floating grounds that did function but it was more of an accident that they did....I've always been taught never to leave floating grounds, especially with electronics. I'm hoping somebody who knows how USB works much better than I do will be able to provide some insight on the risks involved and what I should do. Does anyone know a lot about the insides of a USB cable and how critical grounding is for it's proper function?
Here's the link to the site I'm using for reference. It's translated from russian so read carefully. Thanks.
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http://translate.google.com/translate?hl=en&sl=ru&u=http://ipaq214.ru/2008/11/usbhost/&prev=/search%3Fq%3DFB112AA%2Bpinout%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-US:official%26hs%3DyvL)
Al_B on 7/4/2010 at 21:20
Is this meant to be in comm chat?
Technically a USB cable should have an aluminum shield and a drain wire (which is probably the one you're referring to) and a braided outer shield for anything above low speed devices connected to the outer metal of the USB connectors. Making a cable as described in the article almost certainly wouldn't be certified by the USB organisation and may have problems with other compliance testing.
In practice, the signals should be referenced to the ground power connection so you won't have a "floating ground" in this case exactly. It could problems - but I suspect your experience would be like the author's and you won't see any problems in practice if you leave it as-is.
RocketMan on 7/4/2010 at 22:59
Quote Posted by Al_B
Is this meant to be in comm chat?
Technically a USB cable should have an aluminum shield and a drain wire (which is probably the one you're referring to) and a braided outer shield for anything above low speed devices connected to the outer metal of the USB connectors. Making a cable as described in the article almost certainly wouldn't be certified by the USB organisation and may have problems with other compliance testing.
In practice, the signals should be referenced to the ground power connection so you won't have a "floating ground" in this case exactly. It
could problems - but I suspect your experience would be like the author's and you won't see any problems in practice if you leave it as-is.
Hey Al_B I was actually expecting you to respond :D I forgot to mention the shield....but that's for capacitive coupling is it not? I would imagine that that is of somewhat less importance for what I'm doing. Sorry if I confused my terms. The uninsulated wire which I called the ground is the one you called the drain. There is of course a ground among the 4 usb wires but I wasn't sure whether it was at the same potential as the drain.
Because I was excited to try this thing out I finished the cable before I was able to get your reply :( but I ended up soldering the drain to the 24x chassis anyway since I believed it should be done. I had tested the cable when it was still in pieces, without the drain and it worked but that didn't surprise me too much. In fact for all intents and purposes I can't think of a scenario in which the drain would be used, especially when most USB slave devices like sticks for example do not have their own power supplies. Perhaps if I was plugging into a digital camera or an external hard drive, which has it's own power, then I'd need to worry about that drain actually doing something...am I on the right track?
So far I'd tested 2 sticks and a usb mouse. All work great (although the mouse has no cursor, presumably because there isn't any software to support a cursor...the buttons work and the x,y exists because when i click, whatever the mouse was pointed at activates, I just can't see what i'm clicking)
Al_B on 8/4/2010 at 00:16
Excellent - glad you got it working.
The drain wire is there to help you make a good connection to the shield (as you did by soldering). Normally you'd only terminate a shield at one end to avoid possible ground loops but the USB spec does say that it should be connected to both connectors - leaving it up to the equipment you plug in to decide how to terminate it.
Again, normally the shield is there to help with noise suppression and immunity. For your cable it shouldn't really matter given the distances and environment you're likely to use it in.
