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4-10 Series 90™-70 Enhanced Hot Standby CPU Redundancy User's Guide
–
May 2000 GFK-1527A
4
Programming a Data Transfer from Backup Unit to Active Unit
Optionally, the program logic can be used in both CPUs to transfer eight bytes (4 registers) of data
from the backup unit to the active unit before the next logic solution.
To initiate this transfer, the backup unit executes SVCREQ #27 (Write to Reverse Transfer Area).
This command copies eight bytes of data from the reference in the backup unit specified by the
PARM parameter. Note that SVCREQ #27 only works when its CPU is the backup unit. When its
CPU is the active unit, SVCREQ #27 has no effect.
The active unit stores the transferred data in a temporary buffer. The program in the active unit
must include SVCREQ #28 (Read from Reverse Transfer Area), which copies the eight bytes of
data from the temporary buffer to the reference specified by the PARM parameter. SVCREQ #28
only works in the active unit. It has no effect when its CPU is the backup unit.
There is always a one-sweep delay between sending data from the backup unit using SVCREQ #27
and reading the data at the active unit using SVCREQ #28.
This data copied from the buffer is not valid in the following cases:
during the first scan after either unit has transitioned to RUN;
while the backup unit is in STOP mode;
if the backup unit does not issue SVCREQ #27.
The data should not be used if REM_RDY is off or if REM_RDY is transitioning to on.
Data Transfer Example
The following rungs would be placed in the program logic of both units. In this example, the
backup unit would send %P0001 through %P0004 to the active unit. The active unit would read
the data into %P0005 through %P0008. %P0001 through %P0004 on the active unit and %P0005
through %P0008 on the backup unit would not change. %T0002 would be set to indicate that the
operation was successful and that the data could be used.
REM_ RDY
REM_ACT
SVC
REQ
FNC
PARM
CONST
00027
%P00001
CONST
00028
%P0005
%T00001
REM_RDY
LOC_ACT
SVC
REQ
FNC
PARM
%T00001
%M00001
%T00002