Chapter 6. Service Request Function276 PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual GFK-2950C6.1 Operation of SVC_REQ FunctionPACSystems supports the Service Request function in LD and FBD.6.1.1 Ladder DiagramWhen SVC_REQ receives power flow, it requests the CPU to perform the specialservice identified by the FNC operand.Parameters for SVC_REQ are located in the parameter block, which begins at thereference identified by the PRM operand. The number of 16-bit references requireddepends on the type of special controller service being requested. The parameterblock is used to store both the function's inputs and outputs.SVC_REQ passes power flow unless an incorrect function number, incorrect parameters, or out-of-range references are specified. Specific SVC_REQ functions may have additional causes for failure.Because the service request continues to be invoked each time power flow is enabled to thefunction, additional enable/disable logic preceding the request may be necessary, depending uponthe application. (For example, repeated calling of SVC_REQ 24 would continually reset a module,probably not the intended behavior.) In many cases a transition contact or coil will be sufficient.Alternatively, you could use more complex logic, such as having the function contained within a blockthat is only called a single time.OperandsNote: Indirect referencing is available for all register references (%R, %P, %L, %W, %AI, and %AQ).Operand Data Type Memory Area DescriptionFNC INT variable orconstantAll except %S - %SC Function number; Service Request number. Theconstant or reference that identifies therequested service.PRM WORD variable All except flow, %S - %SCand constantThe first WORD in the parameter block for therequested service. Successive 16-bit locationsstore additional parameters.ExampleWhen the enabling input %I0001 is ON, SVC_REQ functionnumber 7 is called, with the parameter block starting at%R0001. If the operation succeeds, output coil %Q0001 is setON.
