So in your case if the LAN cable between the two servers is cut then both I/O Servers will start communicating to the PLCs. Under almost all conditions this will cause no problem to your system, it may reduce the performance as both I/O Servers will be reading data from the PLCs. So the I/O Servers will be reading twice the amount of data required which may slow down your system response time.

Under some special conditions you may require that only one I/O Server is communicating to a PLC at once. You can do this by using the cicode function UnitControl() (Now called IODeviceControl() in version 3.0 and greater) to manually control the primary / standby switch over. You will need to put a watch dog register into the PLC which the Primary I/O Server will increment at say a 10 second period. The Standby I/O Server can then monitor this register and if it does not increment within 20 seconds can assume that the Primary I/O Server has lost communication to the PLC and it should enable communications. In this way you are using the PLC as the method of communication to verify that the Primary is talking correctly rather than the LAN.

See also Q1138.