For computer systems in general including PLCs, hot swapping refers to the process of adding or replacing components in a control system or a PLC without having to shut down power to the system. In a PLC, this typically refers to I/O modules that plug into the PLC chassis.
For PLCs that don’t have hot swap capability, this means that adding or replacing modules must be done with the power off. (As you might expect, this is called cold swapping.) If not, there’s a high risk of damage to the modules as well as the processor and the PLC chassis.
Hot swapping has clear benefits including simplifying the processing of making modifications to PLCs systems and avoiding larger system shutdowns and slowdowns in production.