USB connector - vs - current supply

[kewakl]

I have a verizon USB720 (qualcomm 3G CDMA) broadband access pod.
The cable had two USB (A) plugs on one end and a USB (A) receptacle on the other end.
The reason for the two plugs on the one end is that you can get more than 500mA with both plugs inserted into two USB ports.

Is this simply because of the parallel cable connection, or MUST the USB device request more current from the hub?
I am somewhat familiar with the initialization current limit of 100mA and the requested operating limit of 500mA,
but am unsure how this device actually has access to the full <1000mA.

Does anyone here know how this functionality is obtained?
Can it be obtained by a powered hub?

I want to have ONE connection supply my programming signals and the 5V, 3.3V and 3.0V(as required) for PICAXE development.
To that end, I am looking to make an adapter that can mix these signals/voltages for my dev boards.

Thanks.
k

 

[inglewoodpete]

The question is: "Is the cable a smart one or a dumb one?"

For the USB power source to supply 500mA, the slave must request it in a dialogue. If the cable is a simple dumb one, then only one of the plugs can have the data leads connected. As a result, the maximum supply is 600mA (negotiated 500mA + simple 100mA).

For a 'smart' cable, it could be possible to get 1A (500 + 500 mA). While I've never seen such a cable, I guess they exist.

I really can't comment on powered USB hubs but I guess there are dumb and smart ones of those too. It is possible that cheapies could supply everything the wall wart was capable of - I just don't know the answer. Check the specs of the one you use.

 

[kewakl]

Thanks IWP,
I've not really understood how this works.
I just have the cable. It *SEEMS* to work as described!
Cannot say that it requests or gets +600 mA.

 

[hippy]

See my full reply in ....

http://www.picaxeforum.co.uk/showthread.php?t=13882

where the important point is that the 500mA USB supply maximum is 'per host controller' not 'per USB socket'.

To draw greater than 500mA the two USB cables must be connected to USB sockets which are on separate host controllers. When connected to the same host controller the device will ask for 500mA, then another 500mA and the host controller should refuse to do so.

A powered hub may have a single host controller and can provide 500mA in total, shared across USB sockets. Some powered hubs may provide 500mA per socket. You need to read the manual but the walwart PSU is usually a good indicator; a 5V/2A PSU for a four port hub is usually an indicator of supporting 500mA per socket.

An unpowered hub, or a powered one with PSU disconnected will only provide 500mA shared by all sockets.

Properly designed USB interfaces which use more than 500mA should be safe because they comply with the USB specification. Any connection to USB which is not in accordance with the USB specification may lead to damage of the host controller or motherboard.

 

[kewakl]

I really do not *want* to get USB 5V.
This thread was somewhat influenced be Alex's post and I didn't want to hijack his thread..
I do have a KYBD connector, but I need to research the laptop's KYBD port current capability.
Then I could *mostly* eliminate wallwart/battery from the dev process.
I understand some projects will still require the warts/batts....motor...lamp...led...radio the list goes on.

 

[hippy]

I do have a KYBD connector, but I need to research the laptop's KYBD port current capability.

Finding the limit may be a case of trying it and seeing but that's walking close to destructive testing. It probably varies from PC to PC, some might take the PSU 5V straight out, it may be fused, it may not.

It would probably be safe to say it's whatever the original PS/2 design allowed for as it would have to be accepted some keyboards may have drawn up to that limit so subsequent designs have to tolerate such a keyboard, but theory might not meet practice.

Added: A quick search reveals the general consensus is a maximum of 275mA for mouse plus keyboard combined.

 

[kewakl]

Finding the limit may be a case of trying it and seeing but that's walking close to destructive testing. It probably varies from PC to PC, some might take the PSU 5V straight out, it may be fused, it may not.

Doesn't matter now, a quick look at the laptop reveals a farily simple case of non presence of a KYBD port. On the DELL laptop, I was mistaking the S-VIDEO port for KYBD.
The HP Omnibook has a KYBD port and a real Serial port.(the serial port is it's saving grace!)
I will try to look for the HP specs!
Thanks for the replies!