Modules And Accessories
3.15
CPU-700-D3R - Redundant CPU Module
Part Number
CPU-700-D3R - CPU module with 23 Kbytes of non-volatile memory for user configuration and a
microcontroller of 15 MHz with real time clock and remote I/O (For CPU redundancy).
Introduction
The redundancy with the CPU-700-D3 is based on a hot-standby mechanism. Therefore, just one
CPU runs the ladder logic as well as the communication with HMI at a specific moment. The active
CPU scans the Remote I/O module while the passive CPU monitors the status of the active CPU
through another port.
However, there is communication between the CPUs to keep the synchronization of the
configuration and also to provide fresh dynamic data to the passive CPU. For example: the Modbus
Variables.
The algorithm responsible for choosing the active CPU tries to minimize the number of control
switches; therefore, if a CPU runs as active, it remains in this state, unless something happens to
put this CPU in worse conditions.
Terminology and Initial Descriptions
Main and backup CPUs
The redundancy status depends on a lot of items as: configuration of the CPU, communication
status through SSIO, communications with the remote I/O modules RIO-700-D3 (RIOs). This
status defines whether the CPU will run the ladder logic as well as scanning the RIOs.
The CPU may assume two roles: Active and Passive. The active CPU runs the ladder logic and
scans the MODBUS variables. The passive CPU periodically checks the active CPU in order to
check if it is necessary to take control.
The configuration of CPU role is set through the rotary switch in the CPU module:
- Rotary Switch in the position 0 (Main): When both CPUs are powered-up at the same time and,
the algorithm checks the rotary switch position (of both CPUs) to choose which CPU is active and
which one is the passive.
- Rotary Switch in the position 9 (Backup): The CPU set as backup grants the control to the other
CPU their current status is both passives and both actives. This configuration is used only when the
CPUs are simultaneously powered-up and when both are passive or active at the same time before
the last power-down.
Configuration Transference
During the power-up procedure, the configuration of the active CPU is passed to the passive CPU.
This transference is always made from active to passive and never in the contrary direction and it
uses the Synchronous Serial I/O (SSIO) Port of the CPU. This synchronous serial communication
channel is used exclusively to transfer the configuration.
During this process two new terms need to be defined:
- Sender: This is the CPU responsible for the communication. The sender CPU handles the
configuration transference between the CPUs.
- Addressee: This is the CPU that receives the configuration.
Configuration synchronism between the CPU’s
This is accomplished in two main phases:
Phase 1- Configuration Transfer
When a sequential power-up procedure takes place, the first CPU powered-up becomes the active
CPU and it transfers the configuration to the other one.
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