Chapter 4. Ladder Diagram (LD) ProgrammingGFK-2950C February 2018 1034.5.1 Do I/OWhen the DO I/O (DO_IO) function receives power flow, it updates inputs or outputsfor one scan while the program is running. You can also use DO_ IO to update selectedI/O during the program in addition to the normal I/O scan.You can use DO_IO in conjunction with a Suspend IO (SUS_IO) function, which stopsthe normal I/O scan. For details, refer to Suspend I/O.If input references are specified, DO_IO allows the most recent values of inputs to beobtained for program logic. If output references are specified, DO I/O updates outputsbased on the most current values stored in I/O memory. I/O is serviced in incrementsof entire I/O modules; the PLC adjusts the references, if necessary, while DO_IO executes. DO_IOdoes not scan I/O modules that are not configured.DO_IO continues to execute until all inputs in the selected range have reported or all outputs havebeen serviced on the I/O modules. Program execution then returns to the function that follows theDO_IO.If the range of references includes an option module (HSC, APM, etc.), all the input data (%I and %AI)or all the output data (%Q and %AQ) for that module are scanned. The ALT parameter is ignoredwhile scanning option modules.DO_IO passes power to the right whenever it receives power unless:■ Not all references of the type specified are present within the selected range.■ The CPU is not able to properly handle the temporary list of I/O created by the function.■ The range specified includes I/O modules that are associated with a Loss of I/O fault.WarningIf DO_IO is used with timed or I/O interrupts, transitioncontacts associated with scanned inputs may notoperate as expected.Note: The Do I/O function skips modules that do not support DO_IO scanning:IC693BEM331 90-30 Genius Bus ControllerIC694BEM331 RX3i Genius Bus ControllerIC693BEM341 90-30 2.5 GHz FIP Bus ControllerIC693DNM200 90-30 DeviceNet MasterIC695PBM300 RX3i PROFIBUS MasterIC695PBS301 RX3i PROFIBUS SlaveIC687BEM731 90-70 Genius Bus ControllerIC697BEM731 90-70 Standard Width Genius Bus Controller
