The following figure shows the basic configuration.
When a control system Ethernet module is faulty or disconnected, the system is switched at a disconnection and the module is connected to a standby system.
In this configuration, when a LAN card of a personal computer, hub, or a LAN cable between them is disconnected, the communication between the computer and a programmable controller will be disabled.
To prevent this problem, insert two LAN cards into the personal computer and configure two Ethernet systems as shown in the figure below.