PFIP is controlled by a 4ms microprocessor controller mounted on the outer top end ring of the WHT. The 4ms is a FORTH based system, which is attached to the WHT utility network, and accepts and sends messages over the utility network conforming to the local standard protocol. The 4ms in turn controls two Local Instrument Motor Positioning Interface Terminals (LIMPITS), one of which controls the stepper motors and barcode reader of the PFIP, and the other of which controls the mechanisms of the atmospheric dispersion compensator. The Limpits replace the Stepper Motor Drive Modules (SMDMs) found in earlier instruments (such as the Cassegrain instruments).