The S7-400H consists of two redundant configuration subsystems synchronized via fiber optic cables. These two subsystems form a fault-tolerant automation system that operates using a dual-channel (2-to-1) architecture based on the principle of "active redundancy." The redundant system architecture is shown in the figure below: The hardware modules required for this system installation are as follows: 1. S7-400H system components: 1 UR2-H rack (racks UR1, UR2) 2 PS407 10A power supply units 2 H-CPUs 4 synchronization modules (used to connect two CPUs, installed in the CPUs and interconnected via fiber optic cables) 2 fiber optic cables 2. One ET 200M distributed I/O device with an active backplane bus, equipped with 2 IM153-2 1 SM321 DI 16 x DC24V digital input module 1 SM322 DO 16 x DC24V digital output module 3. All necessary accessories, such as PROFIBUS. Cable Instructions: 1. The rack number is set via a switch on the back of the CPU (default is 0). Incorrect setting will affect online communication, and the CPU may fail to start properly. 2. Be sure to interconnect the two upstream synchronization modules and two downstream synchronization modules of the CPU using fiber optic cables. Before powering on or connecting the system, ensure the fiber optic cable is connected to both CPUs; otherwise, both CPUs will execute the user program as the master CPU. 3. Connect the programming device to the first fault-tolerant CPU (CPU0), which will become the master CPU of the S7-400H. 4. After powering on, perform a high-quality RAM test, which takes approximately 10 minutes. During this time, the CPU cannot be accessed, and the STOP LED will flash continuously. If using a backup battery, this test will not be performed on subsequent power-ups. Master/Slave Assignment: Generally, both CPUs have equal privileges, so either CPU can act as either the master or backup CPU. 1. When the S7-400H is first powered on, the CPU that starts first enters master mode, while the partner CPU enters backup mode. 2. When both CPUs are powered on simultaneously, the preset master/backup assignment will be maintained. 3. The master/standby CPU allocation will change in the following situations: the standby CPU starts before the master CPU (with an interval of at least 3 seconds); in redundant system mode, the master CPU fails or enters STOP mode; no error is found in ERROR-SEARCH mode. Synchronization: The master and standby CPUs are linked via a fiber optic cable, and this link maintains the execution of the event synchronization program. Master-slave CPU synchronization occurs in the following situations: direct I/O access; interruption; updating user timers; data modification by communication functions. Siemens uses event synchronization, with a synchronization cycle slightly shorter than timed synchronization. The link backup process for automatic CPU synchronization is shown in the following diagram: The system status and operating status of the S7-400H and CPU are shown in the table below: Notes: 1. If startup is successful, the primary CPU (CPU0) will switch to single-mode and execute the user program independently. During the transition to LINK-UP system mode, no blocks can be opened via the "Monitor" option, and no variable tables can be activated. 2. If the standby CPU (CPU1) requests LINK-UP, the primary and standby CPUs will compare their user programs. If differences are found between the user programs, the primary CPU will update the standby CPU's user program. 3. After successful linking, the update begins, and the primary CPU updates the standby CPU's dynamic data (including inputs, outputs, timers, counters, bit memory, and data blocks). After the update, the memories of both CPUs will have identical contents. 4. After the update, both the primary and standby CPUs are in RUN mode, and the two CPUs process the user program synchronously. 5. When the system is in single-mode, linked, updated, or redundant states, the primary CPU is in RUN mode and executes the user program in single-mode. 6. When running in redundant system mode, both the primary and standby CPUs are always in RUN mode. The two CPUs execute the user program synchronously and perform checks on each other. In this mode, breakpoint testing of user programs is not allowed, and interconnected modules must be a pair of identical modules. 7. HOLD mode has a special function and can only be used for testing purposes. In this mode, the S7-400H CPU behaves identically to a standard S7-400 CPU. The fault-tolerant CPU cannot perform linking or update operations, and the standby CPU remains in STOP mode and outputs a diagnostic message. 8. ERROR-SEARCH mode can only be entered from redundant system mode. When the system is in redundant state, the self-test routine compares the primary CPU and the standby CPU and reports an error when a difference is found. During troubleshooting, the system exits redundant system mode, and another CPU becomes the master and continues to operate in single mode. I/O control: At any given time, the fault-tolerant system uses only one interface from the slaves. The active interface is indicated by the ACT LED on the corresponding IM153-2 or IM157. In S7-400H, the path for I/O through the currently active interface (IM153-2 or IM157) is called the active path, and the path through the other interface is called the passive path. The DP cycle is always active on both channels. However, only the input and output values ​​of the active channel are processed in the user program or output to I/O. Features: 1. Smooth master-slave switching, switching time <100ms, output is maintained during switching, information or alarms/interrupts are not lost, achieving bumpless switching. 2. Automatic event synchronization, ensuring that all information, alarms or interrupts are not lost. 3. During download, the program is automatically copied to both CPUs. 4. Online maintenance, all components can be replaced at will in running state, and the CPU automatically enters redundant state after maintenance . 5. All redundant functions are handled by the CPU and do not require user intervention. 6. The distance to the main controller can be up to 10km. I/O Redundancy 1. The I/O redundant signal templates in the central and expansion devices are installed in pairs in the CPU 0 and CPU 1 subsystems, as shown in the figure below: When a CPU loses power due to a fault, the connected I/O modules are considered faulty and cannot be recognized. 2. In a single-sided DP slave station, I/O redundant signal modules are installed in pairs in the ET200M distributed I/O device with an active backplane bus, as shown in the figure below: When one of the slave stations' PROFIBUS connections is lost or the master station's CPU loses power due to a fault, the connected I/O modules are considered faulty and cannot be recognized. 3. In the switchable DP slave station, redundant I/O signal modules are installed in pairs in the ET200M distributed I/O device with an active backplane bus, as shown in the figure below: In this mode, even if the I/O slave CPU fails, the availability of the I/O module will not be affected. 4. Standalone Mode H: I/O redundancy in the CPU, the connection method is shown in the following diagram: I/O redundancy cannot be implemented when the main station CPU shuts down due to a fault. Scan the QR code → Click to follow us! More exciting content awaits you!