Operating States and Modes

Description of the Hot Standby States

The hot Standby states are:

Run Primary CPU

The primary CPU PLC executes the application program and updates the remote I/Os. If a standby CPU is present, the primary CPU sends application data and I/O to it.
Run standby CPU

During each cycle the PLC:
- checks that a primary PLC exists
- checks that there is no command from the primary PLC
- indicates to the primary CPU that it is running well and is ready to take over the process if the primary CPU stops
- checks that there is no CPU, Copro or CRP mismatches (unless allowed)
Local I/Os are updated, but not the Remote I/Os.

For a Quantum Ethernet I/O configuration, the standby PLC checks the RIO Drop connected.
Run Offline

Depending on the setting of Behavior of the CPU in Run Offline mode the PLC executes:
- all sections of the MAST task application program but the I/O is not written
- the first section of the MAST task of the application program but the I/O is not written
- none of the application program MAST task
This state is either manually activated or by the CPU, which detects the state by itself.

If there is no primary PLC, the CPU tries to change to the Run Primary CPU state.

If the primary PLC exists, the PLC checks each cycle to see if can go to the Run standby CPU state. There are several commands available:
- Application transfer
- Any online command
- STOP command
- HALT command
Stop (Offline)

The PLC neither executes the application program nor controls the process. It is not part of the Hot standby system. Two commands are available:
- Application transfer
- RUN command
- Init

The run offline and the stop offline state can occur in the primary CPU and in the standby CPU at the same time.

Table of States

The following table shows the possible states of the 2 controllers of a Hot Standby configuration:

		Controller A state
		Run Primary	Run Standby	Run OffLine	Stop OffLine
Controller B state	Run Prim	N/A	Hot Standby active	Hot Standby inactive	Hot Standby inactive
	Run Standby	Hot Standby active I/O processed	N/A	N/A	N/A
	Run OffLine	Hot Standby inactive I/O processed	N/A	Hot Standby inactive I/O not processed	Hot Standby inactive I/O not processed
	Stop OffLine	Hot Standby inactive I/O processed	N/A	Hot Standby inactive I/O not processed	Hot Standby inactive I/O not processed

Description of Run Offline Use Cases

The following table describes the different situations of the Run Offline state:

If ...	Then ...
The primary CPU PLC enters Run Offline state	The standby CPU PLC takes over the process and becomes Run primary CPU
The standby CPU PLC enters Run Offline state	The primary PLC remains in Run mode
The fibre optic link is disconnected	The standby CPU PLC enters Run Offline state
The actual hardware configuration is different from the configuration defined in the project	Either the primary CPU or the standby CPU PLC starts in Run Offline state
A application mismatch occurs	The standby CPU PLC enters Run Offline state
The standby CPU RIO head (CRP) stops operating	The primary PLC remains in Run mode
There is no RIO connection open in either PLC	If there is no RIO connection to either PLC, the PLCs stay in Run/Offline state. If there is no RIO connection to the standby PLC, the standby stays in Run/Offline state. If there is no connection to the primary PLC, the primary goes to Run/Offline state, and the standby becomes primary if it has at least one RIO connection. Otherwise, the standby goes to Run/Offline state.

RUN OFFLINE State Recommendation

In the RUN OFFLINE state the PLC is not configured as a primary nor a standby CPU. This occurs after the Hot Standby system detects an error or Hot Standby OFFLINE mode has been chosen.

In this state, the CPU main actions are:

execution of the code sections depending on the choice in the CPU Executes menu
no data transfer from primary, except for the %SW60 value
address swap management
local IO management

When using communication EFBs, some applications can be affected by the entire code execution.

It is recommended to:

create a boolean variable

cpu_state:=(%SW61.1) AND NOT (%SW61.0);
assign the section or communication block execution to this variable

With such a fix, unexpected EFB communication calls are avoided if the standby CPU goes to an OFFLINE state.

Recovery from RUN OFFLINE when EIO is used

To recover from RUN OFFLINE due to no-Drop condition:

cable the drops
put one CPU of the system in the STOP mode
wait until at least one Drop has opened a connection with CRP Head

To put the system in the RUN mode again, carry out the procedure in Quantum Hot Standby Operating Modes Overview.

Description of the Hot Standby Operating Modes

A Quantum Hot Standby PLC has some restrictions in terms of changing modes. The following figure shows the state diagram of the Hot Standby Quantum system:

NOTE:

A PLC that is in the Run Offline mode cannot go directly to Run Primary Mode
A PLC that is in the Run Primary mode cannot go directly to Run Standby Mode

Automatic Start in Run Option

At a cold start with the Automatic Start in Run option configured, a PLC restarts depending on the operating mode of the other PLC, this PLC's operability and on whether identical applications are present on both PLCs:

If...	Then...
The other PLC is primary, the two applications are identical and this PLC is operating normally	The PLC restarts in Standby mode
The other PLC is primary and the two applications are not identical or this PLC is not operating normally	The PLC restarts in Offline mode
There is no primary and this PLC is operating normally	The PLC restarts in primary mode
There is no primary but this PLC is not operating normally	The PLC restarts in Offline mode

A local abnormal or inoperative operating state will be reported when:

There is a loss of power to the CPU rack
An application program error that generates a HALT state (for example, a blocking software error)
The hardware or firmware of the CPU module becomes inoperative
The CPU-sync link is disconnected

At a warm start, the PLC restarts depending on the previous PLC operating mode (Stop or Run).

If the previous state was Run, the PLC restarts according to:

operating mode of the other PLC
local PLC's operability or non-operability
on whether identical applications are present on both PLCs (refer to the table above)