There is an old inititive out there called OPOS wich is intended for communication between a POS and different hardware. When I worked with POS-applications(a couple of years ago) we used OPOS to communicate with printers, cash drawers, customer displays etc. Microsoft, IBM, Epson and a lot of diffrent companys stood behind this, but that was in the late -90's, early -00's. It might be worth to investigate if it still can be used and have evolved.