State Industries GFK-0827 manual Role Switch Svcreq

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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
ÐÐ↑ÐÐÐ		SVC_ ÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÈÐÐÐÐÐÐÐÐÐÐÐÐ( )Ð
		REQ
CONST ÐFNC
00026
%R00001ÐPARM
	_____

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.

62 Series 90-70 Hot Standby CPU Redundancy User's Guide ± December 1993 GFK-0827