The Type J coding system provides the means for centralizing the control of switches and signals at remote field locations, subject to the safety features incorporated in the wayside signal apparatus and circuits. The system also automatically supplies information on the conditions of track occupancy, etc., existing at the field locations, thus enabling intelligent control from the central or control office.

The Type J system is simplex in operation, that is, the two-wire line circuit carries only one control cycle or one indication cycle at a time. Control cycle transmission from the office to the field takes precedence over indicaton cycle transmissions from the field to the office. For the transmission of both controls and indications, the step-by-step operation of the entire system is driven by the control office equipment. Controls are transmitted by polar codes applied to the line circuit; indications are transmitted by Mark-Space codes that are superimposed by the field locations on polar driving codes originating in the office.

The system can have 9 to 14 control functions and 10 to 16 indication functions per station, based on how many locations are configured on the system, with a maximum of 64 stations.