A programmable controller is disclosed for the purpose of controlling devices such as machine tools and other machinery, and which is often mated with a host computer numerical controller. The programmable controller is configured such that certain user-designated "fast-scan" outputs can operate at much higher speeds than non-designated "normal" outputs, thereby greatly increasing the response time and efficiency of the operations that are controlled by those fast-scan outputs. The programmable controller uses a processor which scans through its user-definable program (including the value of all system inputs and outputs) before it attempts to change the value of any of its outputs. The physical input/output (I/O) interfaces are located on I/O racks, each of which also includes an intelligent bus adaptor. The intelligent bus adaptor includes its own processor which periodically communicates with the processor in the programmable controller. The intelligent bus adaptor's processor controls the fast-scan outputs located on that particular I/O rack within very short time intervals, and allows for a response time that is an order of magnitude faster than the response time of normal outputs of the programmable controller.