USB switches: To keep alive, or not to keep alive, etc...

[Joel Hammer]

Just an update. The camera functions go off with the camera plugged into the
computer whenever there is power to the computer. Even if the computer has
been halted, or just turned on and is rebooting. So, this is a hardware
issue, apparently.

Joel

On Mon, Feb 10, 2003 at 09:26:23PM -0500, Joel Hammer wrote:
> The question:
> 
> Will a usb switch cut off the connection between the computer and the
> peripheral completely, similar to pulling the plug, or do switches have
> a "keep alive" capability which will make the peripheral think it is
> still attached to the computer?
> 
> That's the question. Here is all the verbiage, if you care to know the
> reason why I ask.
> 
> We are going to get a coolpix 4500 for capturing digital images over our
> surgical pathology cutting board.
> 
> This camera will be connected to a computer, to automatically upload and
> label each image and rename the image appropriately.
> 
> I upload digital images easily with my coolpic 2500 using the usb mass
> storage option and a usb cable. The camera mounts as a scsi device. Very
> nice. No special software is needed. The 4500 has the same usb mass
> storage option, so I assume it will work the same.
> 
> The only hitch is that as soon as you plug the usb cable into the
> computer and the camera, the camera more or less switches off all
> functions and you cannot take any pictures until you unplug the camera
> from the computer. Plugging the cable into just the camera doesn't have
> this effect. So, it looks like some message from the usb bus tells the
> camera what to do. I tried an experiment. I disabled assigning an irq
> to the usb device (PCI) in the BIOS, and removed all modules pertaining
> to usb. This had no effect. So, this must be a hardware controlled
> behavior, beyond the ability of software to control. I assume that the
> usb bus sends some low voltage signal to the peripherals to tell them
> they are connected.
> 
> This camera is going to be tethered to a computer with the usb cable
> while in use. I don't want the users to have to unplug the usb cable
> at the computer to be able to take photos, then plug in the cable to
> upload photos, I am thinking that a simple mechanical usb switch box
> would solve the problem. Just throw the switch to take photos, throw the
> switch back to upload. This will work if the box isn't too smart and
> doesn't send some "keep alive" message to the camera when a peripheral
> is switched off. I assume a dumb mechanical switch box wouldn't send
> any "keep alive" message. However, some switch boxes are smart, and do
> send keep alive messages when they are turned off. These are usually the
> better sort, and are electronic, not mechanical.
> 
> I could just end the suspense by going out and getting a switch (about
> $40 for a nice one), but, I would appreciate hearing others experience
> before I spend my employer's money.
> 
> Thanks, Joel
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