In particular, the outputs of the latch 140 are applied to an array of dual channel, optical isolators U4 to U11, as shown in FIG. 4D. Isolators U4 to U11 are provided to isolate the analog chart recorder 36 and the chart edge printer 38, which may be powered by a conventional AC outlet, from the data acquisition unit 12 to insure that signals, including the energizing voltage as would be picked u