A micropower microprocessor digital indicator

The standard method of analogue data transmission in the process control industry for many years has been the 4-20 mA current loop. It was decided to offer a digital variant of this meter which was plug-in compatible with the analogue version. The salient features of the new meter were as follows: full four-digit LCD display with analogue bar-graph; digital display end-points customer programmable for direct readout in engineering units; direct replacement for analogue version; low cost, not significantly greater than the analogue meter movement it would replace; operable over the industrial temperature range; and intrinsically safe. These features combined to require considerable signal processing and I/O capability yet allow operation at less than a milliwatt power consumption. The need to take both power and measurement from a terminal voltage of less than 0.5 V required a novel design approach in regard of the input circuitry which has since been patented