The present invention relates to a process and apparatus for the production of carboxylic acid esters andor biodiesel fuel from feedstocks containing fatty acids, glycerated fatty acids, and glycerin by reactive distillation. The electronic test key generates a signal which effects a variation in a pulse width of an internal control signal to stress the DRAM during a test mode. A second circuit in the cab unit circulates a second medium. To combat problems associated with making off-chip measurements and adjustments of signal duty cycles, comparisons are made between input signals and divided input signals that allow for easy measurement and adjustment of on-chip signals, including clocking signals.