Sequentially to reduce the program diversion chance, the scan time of backup PLC requires to be less than the main scan time. This is proficient with provisional ladder logic that lets big amounts of the ladder program to be left out according to a status bit from the RPM. The backup PLC scan time can be an order of scale shorter than the main PLC with logic bypassing.
Compute the words number to transfer
Data is transferred in pairs of WX/WY throughout the network of remote I/O. The Home base contains of multiple slots of these pairs of WX/WY. Every slot can move 512 discrete memory locations or 32 V memory locations. We will require to recognize how many total slots of WX/WY will be required to transfer all of the identified important variables. This information will decide the virtual bases number that the RPM will correspond to the system.
Discrete variables of C and Y
These points of discrete I/O can be planned 16 to a V memory location or straight to and from memory addresses of WX/WY. Consequently, every slot can move 512 discrete variables.
Variables of loop
Up to eight loop variables may be transferred to V memory using the instruction of PACK in ladder logic. The instruction of UNPACK might be used in the standby PLC.
These registers may be straight transferred to address space of WX/WY. Once a V memory table has been updated fully by the active PLC, the ladder logic can utilize the instruction of MOVW to transfer the data directly to the address space of WY. In the stand-by PLC the reverse process is utilized to transfer the data from the address space of WX to a table of V memory.
The installation planning
The RPM may be set up in either the rack for main or standby PLC or even in an external rack if preferred. The connections of remote I/O from both PLCs are linked to the RPM and the associations to all remote physical bases are linked to the front wiring connector on the 2541.