Hot Standby System State Examples
Original instructions
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Introduction
This topic presents visual examples of several Hot Standby system states. The focus of each example is the condition of the:
In each example, CPU A is the module with its A/B/Clear rotary switch set to “A”; CPU B is the module with its A/B rotary switch set to “B”.
Each example presumes that every other necessary precondition exists for Hot Standby system operation. For example:
All Communication Links are OK for both CPUs
In this example, all Hot Standby system connections are operational:
Communication link
CPU A
CPU B
Hot Standby link between CPU A and CPU B
OK
OK
Ethernet RIO link between CPU A and CPU B
OK
OK
Ethernet RIO connections between CPU and one or more (e)X80 EIO adapter modules
OK
OK 
1
Hot Standby fiber optic link between CPU A and CPU B 
2
Ethernet RIO main ring
In this example, CPU A and CPU B enter the following Hot Standby states:
If this Hot Standby system state arises during:
CPU A and CPU B perform the following roles:
Sequential start-up of CPU A and CPU B
  • The first CPU to start up is primary.
  • The second CPU to start up is standby.
Simultaneous start-up of CPU A and CPU B
  • CPU A is primary.
  • CPU B is standby.
Run-time
  • The primary CPU remains primary.
  • The standby CPU remains standby.
Hot Standby Link is Not OK for both CPUs
In this example, the Hot Standby link is not operational in both directions, from CPU A to CPU B and from CPU B to CPU A. All other Hot Standby system connections are functioning:
Communication link
CPU A
CPU B
Hot Standby link between CPU A and CPU B
Not OK
Not OK
Ethernet RIO link between CPU A and CPU B
OK
OK
Ethernet RIO connections between CPU and one or more (e)X80 EIO adapter modules
OK
OK 
1
Hot Standby fiber optic link between CPU A and CPU B 
2
Ethernet RIO main ring 
X
Indicates a broken communication link
In this example, CPU A and CPU B enter the following Hot Standby states:
If this Hot Standby system state arises during:
CPU A and CPU B perform the following roles:
Sequential start-up of CPU A and CPU B
  • The first CPU to start up is primary.
  • The second CPU to start up enters wait state, because there can be no standby CPU if the Hot Standby link is not operational.
Simultaneous start-up of CPU A and CPU B
  • CPU A is primary.
  • CPU B enters wait state.
Run-time
  • The primary CPU remains primary.
  • The standby CPU enters wait state.
Hot Standby Link is Not OK for One CPU and is OK for the Other CPU
In this example, a one-directional break exists in the fiber optic cable used to implement the Hot Standby link. CPU A receives transmissions from CPU B over the Hot Standby link, but CPU B does not receive transmissions from CPU A over the link. All Ethernet RIO connections are OK for both CPUs:
Communication link
CPU A
CPU B
Hot Standby link between CPU A and CPU B
OK
Not OK
Ethernet RIO link between CPU A and CPU B
OK
OK
Ethernet RIO connections between CPU and one or more (e)X80 EIO adapter modules
OK
OK 
1
Operational Hot Standby fiber optic link from CPU B to CPU A 
2
Broken Hot Standby fiber optic link from CPU A to CPU B 
3
Ethernet RIO main ring 
X
Indicates a broken communication link
In this example, CPU A and CPU B enter the following Hot Standby states:
If this Hot Standby system state arises during:
CPU A and CPU B perform the following roles:
Sequential start-up of CPU A and CPU B
  • The first CPU to start up is primary.
  • When CPU A starts up (after CPU B), it is standby.
  • When CPU B starts up (after CPU A) it enters wait state.
Simultaneous start-up of CPU A and CPU B
  • CPU A is primary.
  • CPU B enters wait state.
Run-time
  • CPU A remains primary and CPU B enters wait state.
    – or –
  • CPU B remains primary and CPU A remains standby.
One Break Exists in the Ethernet RIO Main Ring
In this example, a single break exists in the Ethernet RIO main ring. Although the break occurs in the segment between the two CPUs, in this example, the break could be located at any point along the Ethernet RIO main ring (2). All other Hot Standby system connections are functioning:
Communication link
CPU A
CPU B
Hot Standby link between CPU A and CPU B
OK
OK
Ethernet RIO link between CPU A and CPU B
OK1
OK1
Ethernet RIO connections between CPU and one or more (e)X80 EIO adapter modules
OK
OK
1. RSTP calculates and implements a redundant path between CPU A and CPU B in case of a single break in the Ethernet RIO main ring. 
1
Hot Standby fiber optic link between CPU A and CPU B 
2
Ethernet RIO main ring 
X
Indicates a broken communication link
In this example, CPU A and CPU B enter the following Hot Standby states:
If this Hot Standby system state arises during:
CPU A and CPU B perform the following roles:
Sequential start-up of CPU A and CPU B
  • The first CPU to start up is primary.
  • The second CPU to start up is standby.
Simultaneous start-up of CPU A and CPU B
  • CPU A is primary.
  • CPU B is standby.
Run-time
  • The primary CPU remains primary.
  • The counterpart CPU remains standby.
Two Breaks in the Ethernet RIO Main Ring Isolate One CPU
In this example, two breaks in the Ethernet RIO main ring have the following effects:
The Hot Standby link remains operational.
Communication link
CPU A
CPU B
Hot Standby link between CPU A and CPU B
OK
OK
Ethernet RIO link between CPU A and CPU B
Not OK
Not OK
Ethernet RIO connections between CPU and one or more (e)X80 EIO adapter modules
Not OK
OK 
1
Hot Standby fiber optic link between CPU A and CPU B 
2
Ethernet RIO main ring 
X
Indicates a broken communication link
In this example, CPU A and CPU B enter the following Hot Standby states:
If this Hot Standby system state arises during:
CPU A and CPU B perform the following roles:
Sequential start-up of CPU A and CPU B
  • CPU A starts up as primary.
  • CPU B starts up as standby.
Simultaneous start-up of CPU A and CPU B
  • CPU A is primary.
  • CPU B is standby.
Run-time
  • CPU B remains or becomes primary.
  • CPU A enters standby state.
Two Ethernet RIO Main Ring Breaks Cause CPUs to be Connected to Different Sets of Ethernet RIO Devices
In this example, two breaks exist in the Ethernet RIO main ring, causing the loss of the Ethernet RIO link between CPU A and CPU B. The location of the breaks cause each CPU to be connected to a different collection of (e)X80 EIO adapter modules on the Ethernet RIO main ring. The Hot Standby link remains operational:
Communication link
CPU A
CPU B
Hot Standby link between CPU A and CPU B
OK
OK
Ethernet RIO link between CPU A and CPU B
Not OK
Not OK
Ethernet RIO connections between CPU and one or more (e)X80 EIO adapter modules
OK
OK 
1
Hot Standby fiber optic link between CPU A and CPU B 
2
Ethernet RIO main ring 
X
Indicates a broken communication link
In this example, CPU A and CPU B enter the following Hot Standby states:
If this Hot Standby system state arises during:
CPU A and CPU B perform the following roles:
Sequential start-up of CPU A and CPU B
  • The first CPU to start up is primary.
  • The second to start up is standby.
Simultaneous start-up of CPU A and CPU B
  • CPU A is primary.
  • CPU B is standby.
Run-time
  • The primary CPU remains primary.
  • The standby CPU remains standby.