Chapter 4 NI-DNET Programming Techniques
NI-DNET User Manual 4-8 ni.com
For strobed and polled I/O connections, determination of a valid
ExpPacketRate can be somewhat complex. If you have trouble
estimating an ExpPacketRate value for strobed/polled I/O, set the
PollMode parameter of your initial call to ncOpenDnetIntf to
Automatic. When you use this automatic EPR feature, the
ExpPacketRate parameter of ncOpenDnetIO is ignored for
strobed/polled I/O (ConnectionType of Strobe or Poll), and NI-DNET
calculates a safe EPR value for you. This automatic EPR is the same for all
strobed and polled I/O connections (scanned I/O).
After you start communication, you can use the ncGetDriverAttr
function to determine the value calculated for ExpPacketRate. From that
value, you can then experiment with other ExpPacketRate configurations
using 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 time
taken for each message.
• The InputLength and OutputLength parameters of each
ncOpenDnetIO determine the time needed for each I/Omessage.
• NI-DNET assumes that each strobed/polledI/O device can respond to
its command within 2 ms.
• NI-DNET sets aside a fixed amount of time for explicit messages. This
time depends on the baud rate.
Using I/O Data in Your Application
Appendix A, DeviceNet Overview, explains that the data transferred to and
from a DeviceNet device on an I/O connection is usually processed by an
Assembly Object within the slave device. Input assemblies represent the
data received by NI-DNET from a remote device, and output assemblies
represent data that NI-DNET transmits to a remote device.
To use a device’sI/O data within your application, you need to understand
the contents of its input and output assemblies. You can find this
information in the following places:
• Printed documentation provided by the device’s vendor.
• If the device conforms to a standard device profile, the I/Oassemblies
are defined within the DeviceNet Specification.