A diagnostic circuit is used to test the operation of a complex digital system by transferring operands to and from a plurality of registers (12, 14, 16). In a typical application the registers (12, 14, 16) in operation utilize parallel data transfers. For diagnostic purposes the registers (12, 14, 16) are connected serially and, in response to selected shift commands, data can be shifted either right or left through the registers (12, 14, 16). First and second buses (18, 20) provide serial, bidirectional paths for data transfer between the group of registers (12, 14, 16) and a service processing unit (26). In response to commands generated by the service processing unit (26) the registers (12, 14, 16) can be loaded right or left and read right or left. Whenever data is being read from the registers (12, 14, 16) the output data is recirculated back into the registers such that a complete cycle results in the original state being restored in the registers (12, 14, 16).