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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/O message.

•

NI-DNET assumes that each strobed/polled I/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.

Appendix A, 

, 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’s I/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/O assemblies 
are defined within the DeviceNet Specification.