Abstract: A programmable controller has a main processor and a scanner circuit in a unit that is connected through a multi-channel, multi-drop serial data link to a plurality of remotely located I/O racks. The I/O racks are adapted for connection to I/O devices connected to control a machine or process. The I/O racks each have an adapter module that communicates with a plurality of I/O modules in its rack. The adapter modules in the various racks are connected through the serial data link to a scanner circuit. Using a series of messages, the scanner circuit and the adapters couple both words of I/O status data for low density I/O modules and blocks of I/O status data for high density I/O modules. The scanner circuit also interfaces the main processor to the serial data link, coupling both conventional I/O status data and blocks of I/O status data from the main processor and organizing this data in messages for transmission over the serial data link.