Programmable controller with expandable I/O interface circuitry

In a programmable controller having a processor which repeatedly executes a user control program and an I/O scan program including an I/O scan sequence, and having a fixed number of I/O buses for interfacing the processor with a corresponding number of I/O modules, the improvement therein comprising:

• an I/O address module connected to one of the I/O buses for receiving data from the processor and generating a rack address on a first I/O expansion bus said I/O address module including means for signaling the processor that the I/O address module is connected to said one I/O bus;
• a set of I/O interface modules connected to a set of other ones of the I/O buses and to a corresponding set of additional I/O expansion buses, each I/O interface module being operable to couple data between one of said other I/O buses and one of said additional I/O expansion buses;
• an I/O rack which includes:
• (a) an adaptor circuit connected to the first I/O expansion bus for receiving the rack addresses generated thereon and producing an enabling signal when its preselected rack address is received thereon;
• and
• (b) a set of I/O modules each coupled to one of the additional I/O expansion buses to couple data between devices on a machine being controlled and its associated additional I/O expansion bus when the enabling signal is produced;
• wherein the processor includes means responsive to the signal from the I/O address module for altering the I/O scan program such that a different I/O scan sequence is performed in the I/O scan program's next execution.