A simple AGLCS, The two HMI Server is connected in LAN with two PLCs/RTUs to implement hot backup redundancy. The Input/Output of the PLCs/RTUs is parallel together for a redundant RTU’s CPU and Input/Output configuration. In normal situation, the Primary HMI Server and the main PLC/RTU is active/in charge. When the main HMI server failed, the standby HMI server will take over to communicate/control the active PLC/RTU. In other hand, if the active PLC/RTU failed, the other inactive PLC/RTU will become active and take over the control function of the system.

A system with shared I/O module will remain the benefit of hot backup redundancy feature on the substation’s AGLCS equipment but with an extra cost saving on the I/O module. Instead of needing two sets of I/Oodules, one set of I/O module is shared by two of the sub-station RTUs. Normally only RTUs is configured to have this type of facilities because the cost of
setting up PLCs in the similar configuration is costly. The FEP is connected to the HMI server on the ATC using redundant communication link; which can be Ethernet LAN (10BaseT using UTP cable or 10Base2 using co-axial cable) or isolated RS485 serial link.

The monitoring HMI in the maintenance center is mainly used by the maintenance staff to monitor the status/alarm of the AGLCS equipment. If by any chance that both of the servers are down due to communication link from tower to the FEP located in the tower technical room or monitoring room, the monitoring HMI is designed to be the last resource where the
system still can be control through the monitoring HMI by the maintenance staff. Note that in normal operation, AGL control is not allowed in the monitoring HMI with password protection.