POWER CONVERSION EQUIPMENT-NAMELY, DC-DC POWER CONVERTERS, DC-AC POWER CONVERTERS, NO-BREAK AC STANDBY EQUIPMENT, INVERTERS AND RINGING MACHINES, POWER SUPPLIES; ELECTRONIC TEST EQUIPMENT-NAMELY, PARITY ERROR DETECTORS, RADIO LOOPBACK UNITS, EQUIPMENT FOR VERIFYING ACCURACY AND EFFICIENCY OF PAPER TAPE PUNCHES USED IN CONJUNCTION WITH TELEPRINTING EQUIPMENT, TEST APPARATUS TO DETERMINE THE PERFORMANCE OF TELEPHONE SWITCHED NETWORKS, RF COAXIAL ATTENUATORS AND RF TERMINATIONS, EQUIPMENT FOR LOOPING BACK TEST SIGNALS FROM A SUBSCRIBER INSTALLED UNIT TO VERIFY QUALITY OF THE TRANSMISSION PATH; ELECTRONIC SWITCHING EQUIPMENT-NAMELY, ELECTRONIC EQUIPMENT FOR SWITCHING OR SELECTING TELEPHONE LINES OR TRUNKS, APPARATUS FOR MANUAL OR AUTOMATIC SWITCHING OF SUBSCRIBER TELEPHONE LINES TO GIVE ALTERNATE USAGE, FUNCTION OR PERFORMANCE; DIGITAL EQUIPMENT-NAMELY, DATA INPUT KEYBOARDS WHICH PROVIDE STANDARD OUTPUT SIGNALSFOR OPERATION OF TELEPRINTER EQUIPMENT AND/OR TO PREPARE DIGITAL INFORMATION FOR COMPUTER USE, APPARATUS WHICH TRANSMITS A DIGITAL CODE IN RESPONSE TO A SENSOR OR ON COMMAND AND DECODING DEVICES WHICH RECEIVE THIS INFORMATION AND CONVERT THIS TO APPROPRIATE ALARM OR ACTION REQUIRED