the invention disclosed is useful in an i / o system of a programmable controller and provides a method and circuitry by which control and diagnostic information is exchanged between a control unit of an i / o module and a plurality of input / output points of the i / o module . a control signal is generated in the control unit in the form of sequential pulse frames such that the control information is defined by a series of pulse width modulated pulses . each frame includes a no - pulse time period following the review of pulses to mark the end of a frame . each i / o point receives a control signal of its own and generates a clock pulse in response to each pulse . the clock pulse initiates a sampling of the corresponding pulse and simultaneously initiates return of a diagnostic signal value to the central unit by implementation of firmware in a switch processor . thus , the control signal provides for sampling of its own content and provides for a return of a diagnostic data bit for each control bit .