Chapter 4
NI-DNET Programming Techniques
©
National Instruments Corporation
4-3
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/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.
Scanned Polling
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.
