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Since the poll commands are not synchronized for individual polling, they 
can often be scattered relatively randomly. They can be evenly interspersed 
for a while, then suddenly occur in bursts of back-to-back messages. 
Because of this inconsistency, you should use smaller MAC IDs for smaller 

ExpPacketRate

 values. Since smaller MAC IDs in DeviceNet usually 

gain access to the network before larger MAC IDs, this helps to ensure that 
smaller rates can be maintained during bursts of increased traffic.

Figure 4-4 shows an individual polling example: MAC ID 3 is polled 
every 10 ms, MAC ID 10 every 35 ms, MAC ID 12 every 100 ms, and 
MAC ID 13 every 700 ms. Only the poll commands are shown (not poll 
responses or other messages).

Figure 4-4.  Individual Polling Timing Example

Cyclic I/O connections essentially use the same timing scheme as 
individually polled I/O connections. Each cyclic I/O connection sends its 
data at the configured 

ExpPacketRate

. The main difference is that 

cyclic I/O data is transferred from slave to master, rather than from master 
to slave.

In the DeviceNet Specification, a poll command message is exactly the 
same as a cyclic output message (master to slave data). Since cyclic data 
from master to slave can be handled using individual polling, cyclic I/O 
connections are more commonly used for input data from slave to master. 
For NI-DNET, this means that for cyclic I/O connections, 

ncOpenDnetIO

 

is normally called with 

InputLength

 nonzero and 

OutputLength

 zero.

Just as for individually polled I/O, you should use smaller MAC IDs for 
smaller cyclic I/O 

ExpPacketRate

 values. Doing so ensures that cyclic 

I/O traffic is prioritized properly.

Poll Cmd 3

Poll Cmd 10

Poll Cmd 3

Poll Cmd 3

Poll Cmd 3

Poll Cmd 13

Poll Cmd 3

Poll Cmd 3

Poll Cmd 3

Poll Cmd 10

Poll Cmd 3

Poll Cmd 12

Poll Cmd 3

Poll Cmd 10

0 ms

20 ms

40 ms

80 ms

60 ms