In SLC-500 addressing, what do the designators I and O indicate?

• A. I stands for input, O stands for output
• B. I stands for internal, O stands for external
• C. I stands for instantaneous, O stands for ordinary
• D. I stands for isolated, O stands for offset

Answer: (A)

In SLC-500 addressing, I and O mark the two data areas the CPU uses to interface with field devices. I designates the input data area—the PLC reads the state of sensors and switches from input modules. O designates the output data area—the PLC writes to outputs to control actuators through output modules. This separation keeps the logic clear: read inputs from I, process the logic, then drive outputs through O. For example, I:0/3 reads the state of input bit 3, while O:0/2 sets output bit 2. The other interpretations like internal/external or instantaneous/ordinary aren’t used for this addressing scheme, so they don’t describe what I and O are doing.