A rotary power converter apparatus for coupling power between a prime driver and plural generators. In one embodiment, there is provided a replaceable sub-assembly for use in a machine at various setpoints including a memory and further including upgraded executable instruction suitable for directing the machine to use the replaceable sub-assembly with different setpoints, where the upgraded executable instruction is stored in the memory. Processes for producing these sub-assemblies and for using them to form electro-optic displays are also described. In this way, the printer can suppress the effects of mounting error and can prevent a decline in accuracy when detecting edges of the paper P.