26 DIGITAL INSTRUMENTS, SYSTEMS AND SUBSYSTEMS-NAMELY, BI-DIRECTIONAL TOTALIZERS, FREQUENCY DIFFERENCE DATA SYSTEMS, FORWARD/REVERSE TOTALIZERS, DIGITAL CLOCKS, COMPUTER ADAPTERS, REMOTE READOUT MODULES, DISPLACEMENT PUMP CONTROL SYSTEMS, PCM TAPE RECORDER TEST SYSTEMS, PRINTED CIRCUIT CARD AUTOMATIC TESTER SYSTEMS, CARD READERS AND SIMULATOR UNITS