The invention provides a novel polyisocyanate compound which is useful as a starting material for an optical material having a high refractive index, a low dispersion and excellent optical characteristics, and a process for producing this compound at good efficiency. A normal internal clock signal for use in accessing a memory array is provided, wherein the memory array may contain redundant memory cells that can be accessed during normal operation. The circuitry may determine the value of signal received at a receiver and produce a binary value.