The firmware in HP all-in-one printer/scanners does not support network file systems (SMB, NFS, etc.). That means any scanned files must be saved to a PC (over USB or Ethernet) or a USB thumb drive (disk-on-key). On the other hand, the printer's console (built-in screen and keyboard) often has a "scan to PC" function that can be used to initiate a scan. But the destination must be a computer that has identified itself as being able to receive scan commands. Even though the scanner initiates the scan, the destination computer drives the whole scanning process. That means the computer must be running HP scan software and must be online during the scan. This is not very flexible or convenient for anyone who uses more than one computer or who wants to scan a document while their laptop is not running.
This monitor solves the problem by listening for scan requests from the console of an HP scanner. It understands the HP scanner protocol and can replace the complicated, proprietary scanning software distributed by HP. It is written in bash, a simple, portable shell scripting language, that is available on a wide range of systems, and does not incur a high computational overhead (you don't need a powerful CPU to run it).
The monitor is designed to run on a headless NAS (network attached storage) server. In my house, my NAS server is a cheap SBC (single board computer) with an ARM Cortex-A9 CPU that runs Ubuntu. Many other NAS boxes that are sold commercially now allow users to install their own apps and services in a similar manner. This allows me to save incoming scanned files in a shared directory on the network.
The script is quite simple and not fully featured. It can only save JPEG files at a single resolution and source document size. It can only save a single document per scan. There are probably many other features that other people would like to use, that this does not do. There is no fundamental limitation in the design that prevents me from doing these things, only time.