Programmable controllers have been in use for more than 40 years, providing industrial control processing to manage electromechanical devices. Programmable controllers were developed to replace old-style, relay-based controls. These controllers provide much benefit over relay-based control systems, requiring a lot less space to execute the same amount of control logic. In addition, these controllers are programmable devices that provide more flexibility than relay-based systems and allow changes to be made without extensive rewiring required. Over the years, these controllers have provided numerous technologic advances and additional capabilities. One key advance in technology has been the ability to apply two programmable controllers in a redundant configuration. Russelectric has been incorporating backup redundancy into switchgear control solutions since the early 1970s.

In the late 1980s, Russelectric developed the software to utilize the GE Fanuc 90/30 PLCs in a redundant, “Hot‐Standby” configuration. Each PLC was configured in a rack with localized rack I/O. This design was improved with the use of distributed I/O, which has worked quite well to provide redundant PLC operation utilizing Genius bus communication protocol. An example of this architecture is shown in Figure 1. GE no longer supports the Genius Communication and the 90/30 PLCs, which have been discontinued and are obsolete. Replacement parts and support are becoming harder to find. The 90/30 PLCs have been replaced with the newer RX3i PACs system controllers that utilize unique reflective memory technology. This allows these newer PLCs to be configured in a redundant synchronized configuration with bumpless switchover in the event of a failure or for maintenance and troubleshooting purposes. This unique reflective memory technology keeps the CPUs synchronized with simple configuration and minimal impact to application performance.