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Redundancy
The Remote I/O Scanner can be used with CPU redundancy, bus redundancy, or both. The suitability of redundancy features for any system depends on the requirements of the application. For Series 90±70 PLC CPUs that are release 3 and earlier, some types of redundancy are not supported.
CPU Redundancy
Genius devices are considered to be controlled by the Bus Controller that sends them outputs. In CPU redundancy, two Bus Controllers on the same bus can send control outputs at the same time. The Bus Controllers must use Device Numbers 30 and 31.
BUSBUS
CONTROLLER CONTROLLER
| (DEVICE 30) | (DEVICE 31) |
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a44876
REMOTE DROP
PS S C A
N
N
E
R
Both Bus Controllers automatically receive inputs and fault reports from all devices on the bus that have been configured as being in ªCPU Redundancyª mode.
How the two sets of outputs are handled by devices that receive them depends on whether the devices are set up for Hot Standby or Duplex redundancy. Both are explained below. If the remote drop contains any analog modules, the only form of CPU redundancy permitted is Hot Standby.
Hot Standby CPU Redundancy
A device configured to be in Hot Standby redundancy mode is normally controlled by the Bus Controller assigned to Device Number 31. If no outputs are available from that Bus Controller for a period of three bus scans, the device accepts outputs from the Bus Controller assigned to Device Number 30. If outputs are not available from either Bus Controller, outputs go to their configured defaults or hold their last state. In Hot Standby redundancy, Device Number 31 always has priority, so that when that Bus Controller is on±line, it has control of the outputs.
Duplex CPU Redundancy
A device configured to be in Duplex redundancy mode compares the outputs it receives from the two bus controllers, to determine whether they match. If corresponding outputs are the same, the device sets the output to that state. If corresponding outputs are not the same, the device sets the output to its configured ON or OFF Duplex Default State. If either Bus Controller stops sending outputs to a device, its outputs are directly controlled by the remaining device. Only discrete devices can operate in Duplex redundancy mode; do not use Duplex mode if the analog drop contains any analog modules.
14 | Series 90±70 Remote I/O Scanner User's Manual ± July 1992 |
