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62 Series 90-70 Hot Standby CPU Redundancy User’s Guide – December 1993
GFK-0827
If the switch occurs due to a controlled condition such as toggling the RCM unit selection
switch, forcing a switch in the user logic program, or because of a fault detected by the
PLC CPU, then the switch-over will occur at the beginning of the subsequent sweep.
The delay will be up to 1 sweep with the possibility of an input and an output scan after
failure detection.
Role Switch SVCREQ
The role switch SVCREQ (SVCREQ #26) will cause the units to switch roles (active to
backup and backup to active) on the next sweep if the units are synchronized and the
timing requirements of the role switch request are met. That is, a manual role switch
may not occur within 10 seconds of a previous manual role switch. Role switches due to
failures or resynchronization are always allowed (the 10 second limitation does not
apply). Power flow from SVCREQ #26 indicates that a role switch will be attempted on
the next sweep. Note that power flow does not indicate that a role switch has occurred or
even that a role switch will occur on the next sweep. For example, the units could fall
out of synchronization due to a link timeout before the next sweep which would
prevent the role switch from occurring.
An example of the SVCREQ #26 function block is shown below. SVCREQ #26 function
has three input parameters and one output parameter. When SVCREQ #26 receives
power flow to its enable input, the PLC is requested to perform a role switch. The three
input parameters are the enable input, the service number FNC, and PARM, which is the
beginning reference for the function parameters. An output coil will be set ON if the
operation succeeds.
| _____
|%I00001 | | %M00001
|——|↑|———| SVC_| —————————————————————————————————————————————————————————( )—
| | REQ |
| | |
| CONST —|FNC |
| 00026 | |
| | |
|%R00001—|PARM |
| |_____|
|
A typical application can be to have a switch on a control console wired to %I0001, which
is the input to the SVCREQ #26 function block. When closed, the switch will activate
the SVCREQ #26, causing a role switch between units.
Note that the 10 second limitation allows these SVC_REQs to be in both units so that
only a single switch occurs if the input is seen by both units.