National Instruments RT Series User Manual

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National Instruments RT Series User Manual Manual pdf 25 page image

Chapter 4 LabVIEW RT ProgrammingRT Series DAQ Device User Manual 4-4 ni.comReal-Time ProgrammingThis section provides an overview of real-time programming using yourRT Series DAQ hardware.Running a VI at Time-Critical Priority withoutthe RT Development SystemInstead of using TCP/IP or VI Server to communicate data to a host PCapplication, you can maximize real-time VI performance on RT SeriesDAQ devices by peeking and poking data to shared memory. Peeking, orreading directly from shared memory, and poking, or writing directly toshared memory, are common programming techniques and do not requirethe VI to yield significant processor time. You can create deterministiccontrol loops that run in excess of 1 kHz when you peek and poke datato shared memory. Refer to Chapter 3, Software Overview, of theLabVIEW Real-Time User Manual for more information about TCP/IP andVI Server.Note Windows NT does not support peeking and poking to shared memory on the PC.However, because the shared memory is located on the RT Series DAQ device, WindowsNT does support peeking and poking to this shared memory.Use of Shared MemoryUsing peek and poke VIs to write directly to shared memory offers the bestperformance. Because you can access shared memory both from a hostLabVIEW application and from an embedded LabVIEW RT VI, you canwrite a control loop program that does not have to slow down significantlyor yield to communicate with the host system. Many such examples are inthe examples\RT\RT Control (7030).llb library.Configuration IssuesThe RT Development System downloads the RT Engine to the RT SeriesDAQ hardware and sends configuration information when you reset thehardware. If you make any change to the device configuration informationfrom MAX, you must reset the RT Series DAQ hardware for the changesto take effect.LabVIEW RT also supports virtual channels, SCXI, and DAQ accessorieson the RT Series DAQ hardware, but their descriptions download to thedevice when the device is reset. If you add a new channel or change theparameters of a channel using MAX, you must reset the RT Series DAQ