A stackable container includes top and bottom surfaces; opposing side surfaces; opposing end surfaces; and interlock mechanisms on at least one of 1) the top and bottom surfaces, 2) the opposing side surfaces and 3) the opposing end surfaces. in order to selectively constitute multiple types of basic digital arithmetic circuits that differ from each other. The rider board includes a first signal path including switching matrices coupled to the DUT, a second signal path including bit error rate testing engines, each of the BERT engines being coupled to each other, corresponding ones of the switching matrices, and to the DUT, and a third signal path including Ethernet testing circuits coupled to the DUT. The base can also include a dummy obstacle having a body portion and a base plug portion.