The embodiments of the disclosure provide a monitoring method, a monitoring module, and a mobile terminal for monitoring a programmable logic controller (PLC) connected with one or more monitor targets. The mobile terminal is connected with the PLC, receives selection information input by a user for selecting a target component for the monitor target from a target component list, determines the target component based on the selection information, and associates the target component and the corresponding monitor target based on first input information input by the user. In an aspect, the mobile terminal receives data about the monitor target transmitted by the PLC, and the corresponding monitor target displays a status of the monitor target based on the data. In another aspect, the user is able to control the PLC through the mobile terminal.

A PLC compares previously acquired device data with currently acquired device data when collecting device data on a control target device over three or more times within one polling time, and stores changed device data as changed data when they are different from each other. When transmitting the changed data to a PC in each polling, the PLC compares the changed data with collected device data. When the total amount of changed data is equal to or larger than the total amount of collected device data, the PLC restores collected device data by using the changed data and transmits the restored device data to the PC, and, when the total amount of changed data is smaller than the total amount of collected device data, the PLC transmits the changed data to the PC.