National Instruments NI-DNET User Manual

Contents

National Instruments NI-DNET User Manual Manual pdf 39 page image

Chapter 4 NI-DNET Programming TechniquesNI-DNET User Manual 4-8 ni.comFor strobed and polled I/O connections, determination of a validExpPacketRate can be somewhat complex. If you have troubleestimating an ExpPacketRate value for strobed/polled I/O, set thePollMode parameter of your initial call to ncOpenDnetIntf toAutomatic. When you use this automatic EPR feature, theExpPacketRate parameter of ncOpenDnetIO is ignored forstrobed/polled I/O (ConnectionType of Strobe or Poll), and NI-DNETcalculates a safe EPR value for you. This automatic EPR is the same for allstrobed and polled I/O connections (scanned I/O).After you start communication, you can use the ncGetDriverAttrfunction to determine the value calculated for ExpPacketRate. From thatvalue, you can then experiment with other ExpPacketRate configurationsusing PollMode of Scanned or Individual.The following information is used by NI-DNET to calculate a safe EPR:• NI-DNET assumes that it is the only master in your DeviceNet system.• The BaudRate parameter of ncOpenDnetIntf determines the timetaken for each message.• The InputLength and OutputLength parameters of eachncOpenDnetIO determine the time needed for each I/O message.• NI-DNET assumes that each strobed/polled I/O device can respond toits command within 2 ms.• NI-DNET sets aside a fixed amount of time for explicit messages. Thistime depends on the baud rate.Using I/O Data in Your ApplicationAppendix A, DeviceNet Overview, explains that the data transferred to andfrom a DeviceNet device on an I/O connection is usually processed by anAssembly Object within the slave device. Input assemblies represent thedata received by NI-DNET from a remote device, and output assembliesrepresent data that NI-DNET transmits to a remote device.To use a device’s I/O data within your application, you need to understandthe contents of its input and output assemblies. You can find thisinformation in the following places:• Printed documentation provided by the device’s vendor.• If the device conforms to a standard device profile, the I/O assembliesare defined within the DeviceNet Specification.