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

[Roger Oberholtzer]

I know that with my Coolpix 990 and a TK-based app, I could take photos
under the control of the software. I think this is because the 990 is not
a disk over the USB, but instead is controlled by a protocol, based on a
Fujitsu  chipset, which a number of cameras (not just Nikon) use. I
do not know about other coolpix cameras. They are not all the same. For
more info, see http://photopc.sourceforge.net/

If the USP connection is lost, the device handle goes away. I would
imagine this would upset your software.

And, do you mean USB switch or USB hub? I suspect the USB switch is a
switch in the sense that a network switch is a switch - there is no actual
switch you can control. Not that I have done an exhaustive search for the
thing.

(I am searching for a Serial->USB converter who's Linux device driver
supports ioctls() that wake up when, say, a CD or RTS line change. Oddly,
most device drivers seems to have a return(0) there - meaning they don't
complain when you try, but they don't do what you want either.)

On Mon, 10 Feb 2003 21:26:23 -0500
Joel Hammer <Joel at hammershome.com> 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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