Why don't backends find hp on usb?

[Larry Evans]

On:

http://localhost:631/printers/hp_psc_2110xi

I selected 'Modify Printer' to arrive at:

http://localhost:631/admin/?op=modify-printer&printer_name=hp_psc_2110xi

where I selected 'Continue' to get a page with:

    Device: [ USB Printer #10 ]
            'Continue'

where:

    [ USB Printer #10 ]

was a rendition of an HTML element:

	<TD ALIGN="RIGHT">Device: </TD>
	<TD>
	<SELECT NAME="DEVICE_URI">
	<OPTION VALUE="socket" >
	AppSocket/HP JetDirect
	<OPTION VALUE="hp:/no_device_found" >
	hp no_device_found
	<OPTION VALUE="http" >
	Internet Printing Protocol (http)
     ...
	<OPTION VALUE="usb:/dev/usb/lp0" >
	USB Printer #1
	<OPTION VALUE="usb:/dev/usb/lp1" >
	USB Printer #2
     ...
	</SELECT>
	</TD>

I've got the hp_psc_2110xi plugged into a usb port on the front
of my computer, but I have no idea which of the following:

   usb:/dev/usb/lp0
   usb:/dev/usb/lp1
   ...
   usb:/dev/usb/lp9

it's connected to.  I've tried lp0, lp1, lp9, but for each, I get:

   "Unable to open USB device "usb:/dev/usb/lp9": No such device"

I've also read:

   http://localhost:631/spm.html#6_2

which contains:

   The serial backend lists devices by looking at serial port files
   in the /dev directory

Since:

http://portal.suse.com/sdb/en/2004/05/jsmeix_print-cups-in-a-nutshell.html

indicates backends are in /usr/lib/cups/backend/, I looked there to
find:

  canon
  epson
  hp
  http -> ipp
  ipp -> ../backend-available/ipp
  lpd -> ../backend-available/lpd
  parallel -> ../backend-available/parallel
  smb -> /usr/bin/smbspool
  socket -> ../backend-available/socket
  usb -> ../backend-available/usb

and since:

   	<OPTION VALUE="hp:/no_device_found" >

appeared in the above HTML select element, I assume the
above hp backend is simply a placeholder which should be
replaced with a real backend?  But how is this done?
If it's not a placeholder, then why is it not finding
the hp plugged into the usb port on the front of my computer?

Thanks for any help.

-Larry