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Chapter 6. Service Request Function
6.1 Operation of SVC_REQ Function
PACSystems supports the Service Request function in LD and FBD.
6.1.1
Ladder Diagram
When SVC_REQ receives power flow, it requests the CPU to perform the special
service identified by the FNC operand.
Parameters for SVC_REQ are located in the parameter block, which begins at the
reference identified by the PRM operand. The number of 16-bit references required
depends on the type of special controller service being requested. The parameter
block 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 the
function, additional enable/disable logic preceding the request may be necessary, depending upon
the 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 block
that is only called a single time.
Operands
Note: Indirect referencing is available for all register references (%R, %P, %L, %W, %AI, and %AQ).
Operand Data Type
FNC
INT variable or
constant
PRM
WORD variable
Example
When the enabling input %I0001 is ON, SVC_REQ function
number 7 is called, with the parameter block starting at
%R0001. If the operation succeeds, output coil %Q0001 is set
ON.
276
PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual
Memory Area
All except %S - %SC
All except flow, %S - %SC
and constant
Description
Function number; Service Request number. The
constant or reference that identifies the
requested service.
The first WORD in the parameter block for the
requested service. Successive 16-bit locations
store additional parameters.
GFK-2950C
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