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4-14 Series 90™-70 Enhanced Hot Standby CPU Redundancy User's Guide
–
May 2000 GFK-1527A
4
Switching Control to the Backup Unit
Control switches from the active unit to the backup unit if:
1
.
the active unit has a failure;
2
.
the pushbutton switch on the Redundancy Communications Module is pressed;
3
.
a switch is commanded from the application program.
4
.
the active unit is placed in Stop mode or powered off.
Switching Times
The amount of time needed to switch control from the active unit to the backup unit depends on the
reason for the switch.
If the active PLC CPU fails or loses power, switching occurs after the backup unit determines that
the active unit failed to rendezvous at the synchronization point. Failure to rendezvous may take up
to 2 failwait timeouts (one for each link) to determine. Control does not transfer until both
Redundancy Communications links have been tried unsuccessfully.
If the RCM switch is pressed, or if the application program commands a role switch (see below) or
if the CPU detects a fault, the switch occurs at the start of the next sweep. The delay is up to 1
sweep. There may be an input and an output scan after fault detection. A control takeover due to
failure or loss of power can occur at any time. However, a manual role switch may not occur within
10 seconds of a previous manual role switch.
Commanding a Role Switch from the Application Program (SVCREQ #26)
The application program can use SVCREQ #26 to command a role switch between the redundant
CPUs (active to backup
and
backup to active). The switch occurs on the next sweep if the units are
synchronized.
When SVCREQ #26 receives power flow to its enable input, the PLC is requested to perform a role
switch. Power flow from SVCREQ #26 indicates that a role switch will be attempted on the next
sweep. Power flow
does not
indicate that a role switch has occurred or that a role switch will
definitely occur on the next sweep. The 10-second limitation allows these SVC_REQs to be in
both units such that only a single switch occurs if the request is made by both units at
approximately the same time. The PARM parameter is ignored by SVC_REQ #26; however the
programming software requires that an entry be made for PARM. You can enter any appropriate
reference here; it will not be used.
Example
In this example application, a switch on a control console is wired to input %I0001. In the program
logic, the reference for %I0001 is used as the input to the SVCREQ #26 function block. When the
switch is closed, logic power flows to SVCREQ #26, causing a role switch between the units.
%I00001 %M00001
%R00001
SVC_
REQ
FNC
PARM
CONST
00026
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