<p>This invention relates to a security key for use in an Industrial Process Control System. The invention provides an apparatus for industrial process control comprising a plurality of processor assemblies and a plurality of input/output assemblies in communication with one another via a backplane portion in which the apparatus further comprises a security key connectable to the backplane portion and containing a resistive element corresponding to each processor assembly and having a respective first predetermined resistance and in which access to a first set of control programs on each processor assembly is disabled unless a resistive element of said respective first predetermined resistance is detected.</p>
