A potential upgrade for the front-end electronics, signal digitization, and data acquisition system of the ATLAS central hadron calorimeter for the High Luminosity Large Hadron Collider (HL-LHC) is described. A Demonstrator is being built to readout a slice of the TileCal detector with this system. The on-detector electronics includes up to 48 analog Front-end Boards for processing the PMT signals, 4 Main Boards for data digitization and slow controls, 4 Daughter Boards with high speed optical links to interface the on-detector and off-detector electronics. Two new ReadOut Driver Boards are used for off-detector data acquisition and preprocessing the digital trigger signals.