Working scanners are detected each time you open Scanners configuration. This detection results in the list of all devices responding to host computer at the moment. But active (turned on) status of the device is not enough to identify it as available scanner. Software support for the scanner should also be installed in your system. That is, appropriate driver for particular scanner model is needed.
So, if your scanner is not visible in the Scanners configuration, you should do the following.
Make sure that your device is connected properly via parallel port or USB and that the device is turned on.
If all the above conditions are met, please, check the following.
Since functional components of MFP (printer and scanner) share the same I/O interface (MFP port), the situation of simultaneous access of different "consumer" application to the same MFP port is possible. To avoid possible conflicts, only one of them at a time is allowed to gain control over the device. The other "consumer" will encounter "device busy" response. If the MFP port is claimed by some application, the scanner won't be detected.
To identify the source of the problem, you should open MFP ports configuration and select the port assigned to your scanner. MFP port's symbol /dev/mfp0 corresponds to parallel port 0, /dev/mfp1 relates to port 1 and so on. USB ports start at /dev/mfp4, so scanner on USB port 0 relates to /dev/mfp4 respectively and so forth sequentially. If you've got the only scanner device connected via USB, the most likely it's on USB port 0 regardless of what physical socket is used.
In the Selected port pane you can see if the port is occupied by some other application. If this is the case, you should wait for completion of the current job or should press Release port button, if you are sure that the present port's owner is not functioning properly.
On some modern Linux distributions MFP connected to parallel port can not be identified "on the fly" when it is switched on. Switch MFP device on before Linux boot.
See also: