Rockwell Automation Publication 1715-RM001A-EN-P - June 2019
11
The ControlLogix/1715 SIL 2 System
Chapter 1
Redundancy System Considerations
• You can experience difficulty synchronizing your chassis if you use the
redundant controller at or very near the connection limits.
• A redundant controller uses more memory than the same application in a
simplex system. If you are at or near the memory limit, you can run out of
memory even though the controller reports it has free memory. This is due
to the crossload mechanism.
• Do not use the USB ports of communication modules in the controller
chassis to access the redundant controllers or networks. Use of the USB
ports in either of the controller chassis to go online results in a loss of
communication after a switchover for the device plugged into the USB
port.
• Task scan time is longer when redundancy is enabled. Make sure that you
consider this when designing your safety system. Do not do your testing
and settings based on a simplex system and then make it redundant.
• You should never communicate with the controller in the secondary
chassis other than to obtain diagnostics.
Redundancy System Details
Two 1756 chassis that have the following equipment provisioned the same:
• Chassis size
• Modules in the same slots
• One 1756-RM2 or 17560RM2XT module per chassis
• Up to two controllers per chassis (for safety systems only one is
recommended)
• Up to seven EtherNet/IP communications modules per chassis.
• 1756-RMCx cables
– These connect the RM modules together between chassis
– You can choose one or two cables
– The cables are fiber-optic Cables
– The redundancy module cables are available in three different pre-made
lengths or you can make your own up to 4KM in length:
1756 Power supplies
• Use simplex power supplies or redundant power supplies
• If the 1756-PA75 or 1756-PB75 (non-redundant) power supply is used
with a ControlLogix 5560 Series B or ControlLogix 5570 Series B
controller, the Series B version of the power supply must be used (1756-
PA75/B or 1756-PB75/B)
Controllers
Catalog Number
Cable Length
1756-RMC1
1 meter
1756-RMC3
3 meters
1756-RMC10
10 meters
