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GFK-1527A Chapter 4 Normal Operation 4-7
4
The Start of Sweep Time message transfer repeatedly coordinates the elapsed time clocks (upon
which timers are based) in the redundant CPUs. The system time is continuous as long as one of
the two systems is running. When a switchover occurs, the same time continues to be kept in the
new active unit.
Output Data Transfer to the Backup Unit
After the initial data transfer, both CPUs operate independently until the end of the program logic
solution. Before the output scan starts, a second automatic data transfer occurs. In this time, the
active unit transfers the selected control and output data to the backup unit. This includes the %Q,
%AQ, %R, and %M memories. For discrete data, the status, override, and transition information is
transferred. If point faults are configured, point fault data is also sent.
1
ACTIVE CPU
Housekee
p
in
g
In
p
ut Scan
Send In
p
uts
and
S
y
nchronize
Lo
g
ic Solution
Send Out
p
uts
and
Other Data
Out
p
ut Scan
Windows
and
Run-Time Dia
g
nostics
BACKUP CPU
Housekee
p
in
g
In
p
ut Scan
Receive In
p
uts
and
S
y
nchronize
Lo
g
ic Solution
Receive Out
p
uts
and
Other Data
Out
p
ut Scan
Windows
and
Run-Time Dia
g
nostics
D
A
T
A
D
A
T
A
2
1
2
First Data Transfer Occurs: %I
,
%AI
Second Data Transfer Occurs: %Q
,
%AQ
,
%R
,
%M
The data is transferred in blocks. Each block is checked for data integrity. The backup CPU holds
the transferred data a temporary area until all the data has been received and verified. Then the
backup CPU copies the data into the actual PLC memories. If the full transfer fails to complete
properly, the backup unit disregards the data in the temporary area and instead uses the values it
obtained during its own logic solution.
After the second data transfer, the active and the backup CPUs independently perform their output
scans and run their programmer and system communication windows. They continue to operate
independently until they synchronize again after the next input scan.