US Patent 7893962 Image output system having multiple devices communication-connected in master-slave relation

A printer periodically sends a request inquiry command “Interrupt In” to a digital camera at a high rate. The digital camera returns a printer status request, a print request, a print cancel request, etc., to the printer as a reply to “Interrupt In.” The camera operates as a usual storage class USB device for the printer except that the camera performs the described operation. Therefore, the printer makes a search for a directory in the camera and reads an image file, a print page layout file, etc., required for print on its own initiative.