Abstract: A structural key is checked under program control to control the mode of operation for a processor-based electronic circuit board. A break-away key portion of the circuit board is connected to the main portion along a boundary of frangible links with circuit paths traversing the boundary and carrying logic signals. The presence or absence of the break-away portion is sensed by sensing the bit pattern at an I/O address. If the absence of the break-away portion is sensed, the function of a user-commanded motion block is tested and processor operation is limited to executing functions for the mode corresponding to the absence of the break-away portion.