Re: PCI cards or hub?

On Mon, 5 May 2008 11:13:42 +0100, wildrover.andy@xxxxxxxxxxxxxx (Andy
Hewitt) wrote:

Jaimie Vandenbergh <jaimie@xxxxxxxxxxxxxxxxxxxxx> wrote:

Wait, what? The UPS will be running of its own battery, as will
(presumably) the Mac. It certainly won't be pulling power from the Mac
via USB.

No, it'll need power to the USB just for communications, otherwise the
Mac doesn't know it's on UPS power does it.

Yes, it will - it's not a dead-man-switch connection, the UPS actually
talks data to the computer. Battery life, mains status, depends on
what the UPS mfr makes available.

So if I don't plug in the USB cable, my Mac will still know it's
connected to a UPS?

Let's start again...

You have a UPS. You plug the Mac into the UPS power outlet. The Mac is
now protected from short-term mains outages.

You also connect the USB cable from the UPS to the Mac. The USB cable
allows the UPS to inform the Mac when the power is out, by sending a
data packet that the Mac is listening for. It also allows the Mac to
ask the UPS how much battery is available, and so on. This can be a
direct cable, or indirect through a hub.

The mains power goes out.

If a hub was in use, and it was not powered from the UPS, the Mac will
now no longer be able to monitor/recieve info from the UPS - this
would generally be non-optimal but not fatal.

If the USB comms between UPS and Mac are still in place, the UPS sends
a packet over USB to the Mac saying "I've switched to battery mode".
The Mac then does whatever comes naturally - usually this would be to
not worry until the UPS reports less than 10% battery left, at which
point to shut down.

So at no point does the UPS require 5V@.5A from the Mac's USB cable to
function. Only the data transfer wires are used.

I suppose the UPS *could* attempt to monitor when the Mac has powered
down by watching the USB 5V wire, but I've never seen one that did -
and the UPS doesn't care anyway. It wouldn't be reliable either, since
most mains-powered computers hold 5V on the USB power wire even when
they're nominally powered off.

If I stick it through a hub
that has its own power, it'll lose comms when the power to the USB hub
fails (the hub isn't on a UPS powered socket), and thus the G5 won't
know to shutdown before the battery runs down.

(I would recommend putting the USB hub on a UPS-powered socket, since
it hardly draws any juice and at some point you'll discover that
you've got your mouse and/or keyboard through it, thanks to Murphy)

Hmm, good point.

The best way to ensure this, is to connect the USB link for the UPS to a
USB port that will stay live when only the UPS is powering the system -
i.e. a built-in USB port on the G5.

That's definitely true, but for not for the reason you have in the
first para.

I still don't see why not?

Hopefully that clears it up!

Cheers - Jaimie
--
"Once you adopt the unix paradigm, the variants cease to be a problem - you
bitch, of course, but that's because bitching is fun, unlike M$ OS's, where
bitching is required to keep your head from exploding." - S Stremler in afc
.

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