Chapter 4 NI-DNET Programming Techniques
© National Instruments Corporation 4-3 NI-DNET User Manual
Polled I/O
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/Oconn ections 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.

Scanned Polling

You can set the ExpPacketRate of each polled I/O connection to the same
value used for all strobedI/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 scannedI/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/Oconnections 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. ScannedI/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/Omessages 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/Oconnection.
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 Automatic EPR Feature section later
in this chapter.