The frame rate of a plant control simulator can be as low as 1 or 2 Hz. However, the number of control inputs (knobs, dials, valves, and so on) can be very large. Special hardware may be required to attach the control inputs and multiplex them onto the VME bus. Also, the number of display outputs (simulated gauges, charts, warning lights, and so on) can be very large and may also require custom hardware to interface them to the computer.