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Logic Comparators allow for the testing of chips (ie: TTL, and in some cases CMOS or other types) while the chip is in on a circuit board by powering up the test board. By having both a chip on a board and the same chip in a tester respond to instructions given by the board, the comparator looks to see if the two chips respond in an identical manner. The failure of a chip on a board to match the actions of the one in the comparator is a strong indication of possible failure.
The BK Precision models are easier and more efficient to use. They contain sockets to insert known good chips. This provides for faster and more versatile testing than the HP 10529 series. The BK Precision model 550 tests TTL chips and is occasionally found on eBay. The model 552 tests similar CMOS chips and is rarer than the 550.