Main Board Description

        All of the ports of the 14M2 are used; four are outputs, one for logging, and three for the display, which uses a shift register. Three inputs report the switch positions. Since one of these is always positive, the overload detector makes for an additional state by pulling all three to common.

        Accurate timing is needed for the watt-hour function and logging. A CD4060-based oscillator is used since it needs only one port.

                On the analog side both the current and voltage inputs are connected by jumpers, both to select positive output from the multiplier circuit, and to disconnect the inputs for adjusting the MC1494 zero pots.

        A buffer is used on the current input to avoid loading the shunt resistors. This is not necessary for the voltage input because it is low impedance