By Bob Culley
In the Modicon redundancy, or Hot Standby (HSBY) systems the two PLCs both contain the same programs. The Primary PLC is the one actually running the process, while the Standby unit is waiting to pick up where the Primary leaves off should it fail. The Standby unit is constantly monitoring the Primary unit for health and I/O communications as well as getting its’ state table updated at the end of each logic scan. The Standby unit also sends out a remote I/O heath burst signal to allow the Primary PLC to know that the Standby PLC is capable of taking over should it be required. Most of the inter-PLC checking is accomplished by the HSBY cable (or fiber as the case may be), however some data is transmitted over the remote I/O cable. The remote I/O to I/O cable connection linking the PLCs is crucial to the bump-less operation during the brief milliseconds between the switching from the Primary PLC to the Standby PLC and during the time afterwards that the failed PLC is replaced. The entire purpose of a HSBY system is to continue the process/application running even if a PLC failure should occur. This should carryover to the time of the replacement of the failed unit as well. The system should function as follows: During normal operation, should the Primary PLC fail, the Standby unit takes over the process/application with no interruption. The failed PLC is removed, replaced and powered up, the program is downloaded and this unit now becomes the Standby unit. Even with all of this high technology, the burden of the system remaining in operation during the failed part replacement falls to the HSBY in-line terminator. A passive, relatively low cost part that is in many instances overlooked at the systems’ installation. A fact that typically remains unknown until after a failure, when the system is being repaired and a process/application shutdown occurs. The two PLC I/O processors are interconnected via coaxial cables with a splitter (used as a combiner) and then on to the remote I/O system. When one of the ports of a splitter is opened, such as during a part/cable replacement, the now open port creates a huge reflection of the remote I/O signal. This may, and will most of the time, disrupt the data to the I/O down stream, causing the process/application to shutdown. The function of the HSBY in-line terminator is to prevent this reflection of the I/O signal, thus preventing any issues. It accomplishes this by manually switching a terminator across the splitter port as the cable is disconnected. The terminator contains a micro switch mechanically linked to the F connector connection point. As the F connector is unscrewed and removed, the micro switch closes and puts the terminator in the circuit. Conversely, when reinstalling the F connector, the terminator is removed from the circuit. It is very important to make this connection point the first to disconnect and then the last to reconnect, to maintain the system integrity. It is vital that this operation be performed in this manner. For this reason, Delta Automation, Inc. produces a special HSBY terminator cable assembly. This cable, (unlike the OEM terminator set with two possible connection points), can only be disconnected at the proper location. Also, two high visibility tags are attached indicating this as the proper connection point. These cables are one of the many items inspected and tested during Delta Automation, Inc.’s remote I/O certification process. Ask about our certification process and reports for remote I/O and Modbus Plus networks.