Abstract: An apparatus is disclosed for testing line replaceable units (LRUs) of an avionics system in which the various units of the system exchange information by means of transmitting and/or receiving digital communication words, each word having a predetermined, standardized format. In a transmit test mode of the apparatus, the information content of a test word is formulated in either engineering or binary form by using a keyboard to create the word manually, or by retrieving a preprogrammed word or sequence of dynamically changing words from a plug-in programmable read only memory (PROM). Once formulated and stored, the communication word is repetitively transmitted, at selected word and bit transfer rates, from one or more output ports provided on the test set and applied, by a jumper connection, to a data communication bus that, in turn, is connected to one or more avionic LRUs.