Paul wrote: > The 4-port USB hub I have uses a 5V 2.6A AC adapter, which is a fair > amount of power. With a hub powered by the computer, I think you'd > need to be cautious with devices that are powered by the USB plug, so > a printer is probably low-draw, but charging an iPod would need more > juice. It also depends on the size of the computer's power supply.
The USB spec requires that every device start up with 100mA (@ 5V) maximum (and it also requires that the computer / hub supply 100mA to any device initially). The device can then negotiate with the computer to supply (or have the hub supply) up to 200, 300, 400 or 500mA. This is subject to available power. If a hub is bus powered then the most it can supply is 400mA maximum (100mA is reserved for the hub electronics). A powered hub, provided it's power supply has adequate capacity, can supply 500mA to each device. So for a 4 port hub, 2.1A is sufficient (the extra 500mA is likely due to availability of that particular PS). This is why 4 ports is the max for a bus powered (or optionally bus powered) hub. There isn't enough power from the computer to power to power the hub and more than four ports, even at the minimum load setting. Yes, a printer or most devices with their own power supply won't need any power from the USB port. But the spec still requires 100mA be allocated to such a device. I don't know if it happens but it's possible that devices such as an iPod can run on however much current is available, 100, 200, 300, 400 or 500mA. One of the biggest problems with USB is that many devices violate the spec, such as attempting to draw 500mA or more regardless of what the computer has allocated to them. This is likely one of the major causes of devices not working properly. This is especially true of the class of devices that simply use USB for power and don't have any control electronics that interface with the computer (such as fans, lights and so on). -- Clark Martin Redwood City, CA, USA Macintosh / Internet Consulting "I'm a designated driver on the Information Super Highway" --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed Low End Mac's G3-5 List, a group for those using G3, G4, and G5 desktop Macs - with a particular focus on Power Macs. The list FAQ is at http://lowendmac.com/lists/g-list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to g3-5-list@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/g3-5-list?hl=en Low End Mac RSS feed at feed://lowendmac.com/feed.xml -~----------~----~----~----~------~----~------~--~---
