The system of Hot Standby is compatible with software of Unity Pro, and offers Quantum CPUs with the availability high level required by the applications of critical process, in terms of their control or command system. At the system center are two racks of Quantum PLC, usually recognized as the "Standby" PLC and the "Normal" PLC. Their configurations of hardware have to be equal (similar modules in every local rack). The key part, on each of them, is the processor of 140 CPU 671 60, especially designed for architectures of Hot Standby with software of Unity Pro. This processor is a double-slot module that joins the function of Central Processor Unit with that of the surplus coprocessor in the similar casing.
The “Normal” PLC performs the program application and the I/O controls. The "Standby" PLC keeps on in the background, equipped to take over if required. The “Standby” PLC is linked to the "Normal" PLC through a high speed link of fiber optic (100 Mbps) incorporated in the CPU. This link of fiber optic (62.5/125 multimode) can be expanded to 2 km without any added particular equipment. It is using this that the data of user application is renewed cyclically on the “Standby” PLC. In the occurrence of a surprising failure influencing the “Normal” PLC, the system standby switches over automatically, the application program will be changing execution and the I/O control over to the Standby PLC, with an up to date the context of data. Once they have updated over, the “Standby” PLC turns into the "Normal" PLC. Formerly the faulty PLC has been renovated and reconnected to the system standby; it takes the function of the “Standby” PLC.
Using the system of Hot Standby with software of Unity Pro means a smooth exchange from normal to standby at the outputs. The exchange is apparent for the process, which will carry on to be controlled without any stable ill-effects from the happening of a failure of hardware. The system of Hot Standby with software of Unity Pro thus raises productivity by minimizing downtime.
Functions of Application program memory space
All the space of memory kept for the application program can be controlled by the system of Hot Standby with Unity Pro. The RAM memory for the processor of 140 CPU 671 60, committed to applications of Hot Standby, can be enlarged to 7.168 Mb by the adding of a format PCMCIA memory card with an embedded RAM memory of 768 Kb.
The application program installation does not vary basically from installing an easy PLC program. It fundamentally utilizes the information given by a dedicated dialog box, entered at the stage of configuration.
Mini-terminal on front panel
The processor of 140 CPU 671 60, similar to any Quantum CPU, comes in the dual-slot module form, and provides a mini-terminal at the top of front panel. Prepared with a screen of LCD and browse buttons, it has a unique sub-menu for the system standby. It can be utilized for instance to ensure the PLC machine status, to force it into mode of connected or disconnected, with regard to the system standby.
The standby system control is controlled by an internal register called the Command Register, accepted by a system word. This Command Register admits user requests, stated via the dialog box configuration and/or through the mini-terminal on the front panel. The Command Register can be utilized in special to disable commands acknowledgement made from the mini-terminal. The status feedback of this redundant system is provided by a Status Register, which is also accepted by a word system.
The function blocks standard are made accessible in the programming environment of Unity Pro, making it probable to run in mode of read/write on the Command Register and in mode read on the Status Register, by classifying independently each of the bits carrying a specific function.
The application context cyclic transfer
At the initial of every scan cycle, the data memory content in the "Normal" PLC is transmitted to the "Standby" PLC through the link of fiber optic, at the same time as the tables content that are images states of the input and output. The system of Hot Standby is consequently capable to transfer all the 128 Kb created available to accept the located variables from the "Normal" PLC to the “Standby” PLC. As long as unallocated application variables are concerned and also data application for example DFB instance data, such as, not less than 512 Kb can be transmitted. The principle of substitutes, as well as replace times along with the data volume.
Program discrepancies monitoring
The most applications of redundant PLC guess equal application programs on both CPUs. To this finally, a comparison is created of the user program resident on both PLCs. It gets place immediately on power-up, and is recurring continuously while the system of redundancy stays connected. The "Standby" PLC will cut off itself from the system of redundancy as soon as dissimilarity in program is sensed by default. Sequentially to maximize availability of control or command system, with during interventions on the user program, it is though possible, through the configuration dialog box or via the register of command, to allow the continual connection of the system redundancy with applications whose program database and/or code are different.
Ensuring parity of the PLC memories content
On the secondary PLC rack powering up, the content parity of the memory PLC in relation to that of the main rack of PLC is ensured repeatedly (Plug and Play) in some examples. This is mainly true when this secondary PLC is blank, or even when it includes a different configuration. Finally of the transfer, the system redundancy is linked; the principal PLC then gets the role of “Normal” and the secondary the role of "Standby". The user can also build a request for an improve through the mini-terminal, which can be contacted from the “Normal” PLC front panel, particularly after a modification has been finished to the application. This process on the mini-terminal can be executed by a maintenance engineer, with no requiring resorting to a terminal of programming. This function is also accessible via a bit of Command Register.
Operating systems upgrading
A bit of Command Register, located if required from the dialog box configuration of the system of Hot Standby, is applied for the operating systems sequential upgrading of both PLC machines, while retaining control of the process with the program application.
Communication port addresses automatic exchange
The particular addresses of the equivalent ports communication are substituted automatically on the "Standby" and "Normal" PLCs when the redundancy system changes over. This addresses exchange is unreserved for the ports of Ethernet and Modbus Plus. It happens conditionally for the local port of Modbus on the processor of 140 CPU 671 60. This function significantly simplifies the developer task on the systems of supervisory control (SCADA, HMI, etc). In consequence, a data address therefore illustrates an “operational” PLC ("Normal" or "Standby") and there is no a physical PLC.
Exchange mechanisms automatically during communication
The I/O architectures irrespective of utilized (mixed I/O or RIO remote I/O), the system of Hot Standby repeatedly controls the mechanisms exchange between the I/O and the PLC executing the function of "Normal".