Abstract: A system is disclosed for testing a plurality of items of test data, the system comprising memory means for storing a plurality of items of stored data and, in response to an input signal identifying one of the items of stored data, producing an output signal dependent on that item of stored data. The input signal is dependent on one of the items of test data and on a feedback signal comprising at least part of the output signal produced on testing a previous item of test data, at least part of the output signal providing an indication of the result of the testing.

Abstract: This invention relates to railway track circuits. An active track circuit termination unit (4) is connected to the rails and, via a transmitter unit (4a), transmits signals into the rails to terminate track circuits (A,B). The transmitter unit also transmit data signals, for example track circuit signals, into the track circuits.