The Turnkey Board includes miscellaneous circuitry controlling the operations of PROM, I/O Channel, Sense Switches, Front Panel and Auto-Start logic.  

The System Turnkey Board consists of:
    •   Sense Switches
    •   Prom Memory
    •   Serial 1/0 Channel
    •   Front Panel Logic
    •   Auto-Start Circuit
    •   Miscellaneous Signals

Detailed information on logic and operation of each is shown int the manuals see here and here.
The schematic for the board can be obtained here.