An I/O object is an implicit or explicit language object for an expert function module or a I/O device on a fieldbus. They are of the following types: %Ch, %I, %IW, %ID, %IF, %Q, %QW, % QD, QF, %KW, %KD, %KF, %MW, %MD, and %MF.

The objects' topological address depends on the module's position on the rack or the device's position on the bus.

For Premium/Atrium PLCs double-type instances of located data (%MD<i>, %KD<i>) or floating (%MF<i>, %KF<i>) should be located by an integer type (%MW<i>, %KW<i>). Only I/O objects make it possible to locate type instances (%MD<i>, %KD<i>, %QD, %ID, %MF<i>, %KF<i>, %QF, %IF) by using their topological address (for example %MD0.6.0.11, %MF0.6.0.31).

For Modicon M340 PLCs, double-type instances of located data (%MD<i>, %KD<i>) or floating (%MF<i>, %KF<i>) are not available.