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Polled I/O connections use a separate poll command and response message 
for each device.

The overall scheme that NI-DNET uses to time polled I/O connections is 
determined by the 

PollMode

 parameter of 

ncOpenDnetIntf

. This 

PollMode

 parameter applies to all polled I/O connections (all calls to 

ncOpenDnetIO

 with 

ConnectionType

 of 

Poll

).

The following sections describe different schemes you can use for 
polled I/O.

You can set the 

ExpPacketRate

 of each polled I/O connection to the same 

value used for all strobed I/O. Using a common 

ExpPacketRate

 for all 

strobed and polled I/O is referred to as scanned I/O. Scanned I/O is also 
referred to as scanned polling with respect to polled I/O connections. When 
you use scanned I/O, NI-DNET transmits all strobe and poll command 
messages onto the network in quick succession.

Scanned I/O is a simple, efficient way to handle I/O connections that 
require similar response rates. With scanned I/O, the master knows that all 
strobe and poll commands go out at the same time. Therefore, the master 
does not need to manage individual timers, thus optimizing processing 
overhead. Scanned I/O also provides overall consistency. If a given 
DeviceNet system uses only scanned I/O, you know that all higher level 
control algorithms can execute at the single common strobe/poll 

ExpPacketRate

.

The common 

ExpPacketRate

 for all strobed and polled I/O connections 

should provide enough time for all strobe/poll commands and each slave’s 
response. You must also allow time for other I/O messages and explicit 
messages to occur in the 

ExpPacketRate

 time frame.

NI-DNET provides two different methods you can use to configure 
scanned I/O:

•

If you set the 

PollMode

 parameter of 

ncOpenDnetIntf

 to 

Automatic

, NI-DNET automatically calculates a valid common 

ExpPacketRate

 value for each strobed and polled I/O connection. 

When you use this scheme, you do not need to specify a valid 

ExpPacketRate

 when you open your strobed/polled I/O connections. 

For more information, refer to the 

in this chapter.