@volodymyr wrote:
…when I get CAP_DEVICEONLINE capability , I always get TRUE result…
It sounds like the DS you’re using does not implement CAP_DEVICEONLINE according to the specification. Some DS’s do but some don’t. I remember one that showed an error message on MSG_OPENDS if the device was not there.
You will not be able to detect when the scanner is present if the developer of the DS you are using is unwilling to fix the problem.