The CPU (1) measures the width of the road-speed pulse signal from the speedometer (5) continuously and estimates the width of the next pulse from the widths of the immediate and preceding pulses. It computes a setting for the stepping motor (7) and the number of steps required from the immediate setting. An address time for each step is computed for which the address circuit (6) supplies the current, after which the positional data are renewed and the new position is stored in random-access memory (3).
