Publication 1756-UM001G-EN-P - January 2007
114 Configure Redundancy
System Considerations
ControlNet Considerations
in Redundant Systems
You can have as many as five ControlNet communication modules in
a redundant chassis. You can use 1756-CNB and 1756-CNBR
Controlnet Modules.
You must have at least two ControlNet nodes external to the
redundant controller chassis to avoid timeouts on switchover.
The lowest ControlNet node must be outside the redundant controller
chassis.
Items in Primary and
Secondary Chassis
Consideration
ControlLogix controller As of firmware revision 13, you can use these combinations of ControlLogix controllers in a redundant
chassis:
• one 1756-L55 controller
• two 1756-L55 controllers
• one 1756-L6x controller
When configured for redundancy, the secondary controller automatically receives and buffers data.
A redundant controller uses twice as much data memory and I/O memory space as a nonredundant controller
The controllers synchronize data transfers to support a bumpless switchover.
• A redundant controller has a significantly longer scan time than a nonredundant controller
• The scan time can affect high-speed processes requiring fast (<50 ms) program scans.
• Minimize the scan time impact by packing data into arrays and structures so the data transfer from
primary to secondary controller is more efficient
You do not need a special version of RSLogix 5000 software for redundancy
Communication modules • Only controllers, 1756-CNB and 1756-CNBR modules, 1756-ENBT modules, and one 1757-SRM
redundancy module can reside in a redundant controller chassis
• As many as two EtherNet/IP modules can reside in a redundant chassis.
• As many as five communication modules can reside in a redundant chassis, such as two EtherNet/IP
modules and three ControlNet modules.
• To connect to other networks, bridge through another ControlLogix chassis.
• Use a separate network for HMI and I/O communication.
I/O modules All I/O is remote from the redundant controller chassis.
Redundant power supplies The 1756-PA75R and 1756-PB75R redundant power supplies provide reliable chassis power.
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