The System Controller is a Z-80-based microcomputer running from a 4K PROM. Two RAM ICs are standardly included, but up to six RAM ICs can be added. System Inputs include the control switches, footswitches, tape input (sync tones and data), and the ADC voltage, Vadc, which is multiplexed from each module and processed in the computer by U224 DAC and U227 ADC window comparator, similarly to ADC function in the Rev 3 Prophet-5s. U225 System Bus transciever transfers data from the Module Bus to the Controller data bus. System outputs include the switch and display LEDs, tape output (sync tones and data), Slot Selects (from U108), data via the transceiver, and Vdac, which is strobed to the Module Bus and demultiplexed within installed modules.