Two port implementation for a USB application consuming more than 2.5W

Ask port1 for 500 mA, and only then ask port2 for another 500mA. And report to the enduser what happends. Like led1 red->green, led2 red->green. If the device doesn't get it's way powerwise let it still be present. But leave powerhungry parts offline.

IF the potentials of the usb ports doesn't match. Not that I think any usb device does this.. However a user might plug the connectors to two different devices ;) (gotcha!)

"Brendan Gillatt" wrote in message news: snipped-for-privacy@pipex.net...

Hi Brendan,

This is a reasonable point. As it happens, any indicators that I will use will be software based.

Excuse my ignorance but do you know if people with colour blindness have difficulty distingushing colours (red/green) on computer monitors in the same way as with LEDs, etc.?

Thanks, Kieran

Hi,

Thanks ... I think that this is the avenue that I will go down alright although the idea of using a second USB peripheral IC just to enumerate correctly to get the 500mA doesn't seem very cost effective to me.

I may try to implement part of the second USB interface in some spare capacity I may have available on an FPGA.

A possible potential difference between the two 5V voltage sources is something that will require some thought alright but I have some ideas for this.

Thanks again.

Cheers, Kieran

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There are all sorts of colour blindness'. I have one, and rarely notice it. Some weird text colour schemes, including green type on some sort of background, are very hard for me to see.

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Just ensure that you load the two supplies separately. I.e. don't parallel the two supplies. Since their output voltages will not be equal, the result would be that one supplied all the current.

Yes. Somebody who is red-green color-blind (the most common form of color-blindness) will have pretty much the same problems with a CRT and with LEDs.

Red/green colour blindness most common and most often forgotten I once had a link to web page about various colour pairs that should be avoid for most things especially web pages to avoid these issues.

Another word of caution that everybody might like to remember in some application fields of which medical is one

ONLY use RED indicators to signify events like

- Danger (that does NOT mean power on) - Fault with the equipment - Patient EMERGENCY (e.g. heart stopped)

It seems a shame that the easiest and cheapest LED colour for manufacturers is the one you should use least.