Abstract: Apparatus for controlling an operational test mode of a scannable latch in a test scan chain, the scannable latch comprising a scan latch and a functional latch coupled thereto, comprises: first circuit for gating a clock signal to the functional latch, the functional latch being responsive to the gated clock signal to capture operational data, the first circuit including an input for controlling the gating operations thereof; and second circuit governed by a selection signal to apply a selected one of a first signal and second signal to the input of the first circuit to control the scannable latch between controllable and observable test modes of operation.