Deleted
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Post by Deleted on Sept 23, 2011 4:34:50 GMT
Devices like USB memory sticks can operate from an external +5V supply, without the need to see Vbus +5V from the USB port.
An external DVD writer case operating via USB 2.0, on the other hand , needs to see the Vbus +5V supply even when using an external PSU. Is their any timing involved here, or would just seeing +5V coming from the USB cable be sufficient ?
Alex
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xerxes
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Post by xerxes on Sept 23, 2011 5:26:36 GMT
This is a wild stab in the dark; do DVD RW drives use 12v and 5v like hard disc drives?
Could it be that the external PSU is only supplying 12v and the USB cable supplying 5v?
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Post by Deleted on Sept 23, 2011 5:33:58 GMT
This is a wild stab in the dark; do DVD RW drives use 12v and 5v like hard disc drives? Could it be that the external PSU is only supplying 12v and the USB cable supplying 5v? Hi Owen Some DVD RW drives can operate off external +12V , and presumably also step this down internally for a +5V rail (?) They usually (?) also have a normal 15 pin input socket to accept the normal +12V and +5V. (An RG member's new Plextor external BR writer does this.) In this case, I am using an internal type LG BR writer externally, supplied by both +12V and +5V from the linear PSU. This LG BR writer is rated to use +12V 2A and +5V 2A. Other people in C.A. forum have also reported still needing to see Vbus +5V from the USB port despite using external power. I would like to be able to get away from having the normal noisy Vbus SMPS +5V present, as I can with the Corsair Voyager. Alex |
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Post by gommer on Sept 23, 2011 6:06:10 GMT
Vbus on USB is a real headache. Once, i knew and understood all the possibilities. But the main trouble is that different configurations are allowed.
I'm not sure if you can get away without Vbus, but i'm sure you can at least isolate it with an optocoupler. In practice and in most cases you can feed Vbus locally from the client PS. The signal must be present whatever it's source and its ground will need to be referenced though (isn't D1 and D2 differential and 'floating' within limits?).
I know this is not a complete story, but you'll have to dig USB specs to be sure. Silicon Laboratories has some application notes for MCU's that also explain pretty well what needs to be done WRT USB.
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Post by Deleted on Sept 23, 2011 6:13:15 GMT
Hi Marc
I was thinking of trying to connect the Vbus lead to the +5V from the PSU via a diode, and use a cable like I am presently using for the Corsair with Vbus disconnected at the PC end of the USB cable.
Could be a little tricky to try though.
Yes, but my tests with a C.R.O. suggest that perhaps the differential receivers aren't perfect at rejecting common mode rubbish, as a 100uF low ESR electro across Vbus and earth at the incoming end of the USB cable reduced visible rubbish on Vbus by around 50%.
This rubbish included low level data packets, presumably being coupled from the adjacent D+ and D- twisted pair.They are all within the same shield, but not normally screened from each other.
Regards
Alex
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Post by gommer on Sept 23, 2011 6:18:16 GMT
Well that can't be hard to try and sound like a good plan. Simply slice open a cheap USB cable and you'll know if it electrically works. Once you know, you can continue to do the final version.
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Post by Deleted on Sept 23, 2011 6:25:23 GMT
Well that can't be hard to try and sound like a good plan. Simply slice open a cheap USB cable and you'll know if it electrically works. Once you know, you can continue to do the final version. Marc I already have a couple of modified USB 2.0 cables with new type A plugs fitted at the PC end, and the red Vbus wire folded back and not terminated.Others tell me that there is an audible improvement. They work very well with the Corsair Voyager, but the HDD or Optical drive in an external case is not identified without Vbus pesent. Alex P.S. Others have simply isolated the Vbus at the outgoing socket with a small piece of insulation.  |
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Post by Deleted on Sept 23, 2011 7:05:20 GMT
Another wild thought;
Would putting a dummy load on the vbus help?
i.e. have the 5v line from the PC end spliced out of the cable so that it is still in the A plug but leaves the USB bundle immediately after. Then follows its own seperate wire (I guess some kind of link into ground as well, casing or pin) to the dummy load.
Therefore, in theory, the PC thinks the 5v is being drawn and the drive thinks it's receiving the the 5v that actually comes from the external PSU.
If that does not work then there would have to be some kind of timing/switching link?
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Deleted
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Post by Deleted on Sept 23, 2011 7:37:32 GMT
Another wild thought; Would putting a dummy load on the vbus help? i.e. have the 5v line from the PC end spliced out of the cable so that it is still in the A plug but leaves the USB bundle immediately after. Then follows its own seperate wire (I guess some kind of link into ground as well, casing or pin) to the dummy load. Therefore, in theory, the PC thinks the 5v is being drawn and the drive thinks it's receiving the the 5v that actually comes from the external PSU. If that does not work then there would have to be some kind of timing/switching link? Chris AFAIK, Vbus is only monitored for excess current draw. Alex |
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Post by pcourtney1 on Sept 23, 2011 10:56:13 GMT
maybe something like my external firewire CD housed in a case that allows you to connect the 4-pin non-powered cable version of FireWire —also called "IEEE 1394" ( also branded "i.Link" by Sony) to your laptop or computer, then you know that only the external linear PSU is powering the CD reader for DAE, no messing with USB computer.howstuffworks.com/firewire4.htm |
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