An integrated circuit with a test interface contains a boundary scan chain with cells (14) coupled between a test data input (TDI) and output (TDO) in a shift register structure. Each cell (14) is also coupled between a respective one of the terminals (16) and the core circuit (10). A test control circuit (TAP_C) supports an instruction to switch the boundary scan chain to a mode in which mode selectable first ones of the cells (14) transport data serially along the boundary scan chain while selectable second ones of the cells (14) write or read data that has been or will be transported through the first ones of the cells (14) in the further mode to or from the terminals (16) from or to the scan chain.