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Change-of-State I/O connections use the same timing scheme as cyclic I/O 
connections, but in addition to the 

ExpPacketRate

, COS I/O sends data 

to the master whenever a change is detected.

For COS I/O, the cyclic transmission is used solely to verify that the 
I/O connection still exists, so the 

ExpPacketRate

 is typically set to a large 

value, such as 10,000 (10 seconds). Given such a large 

ExpPacketRate

, 

the main performance concerns for COS I/O are an appropriate MAC ID, 
and if needed, a nonzero 

InhibitTimer

.

In many cases, a given COS I/O device cannot detect data changes very 
quickly. If a COS device is capable of detecting quickly changing data, 
there is a chance that it could transmit many COS messages back-to-back, 
precluding other I/O messages and thus dramatically impairing overall 
DeviceNet performance. This problem is demonstrated in Figure 4-5.

Figure 4-5.  Congestion Due to Back-to-Back COS I/O

This problem can be prevented if you increase the MAC ID of the 
frequently changing COS I/O device. If the COS device has a higher 
MAC ID than other devices, it cannot preclude their I/O messages.

You can also prevent back-to-back COS I/O messages if you set the 

InhibitTimer

 driver attribute using 

ncSetDriverAttr

. After 

transmitting COS data, the I/O connection must wait 

InhibitTimer

 

before it can transmit COS data again. A reasonable value for 

InhibitTimer

 would be the smallest 

ExpPacketRate

 of an 

I/O connection with a larger MAC ID than the COS I/O device.

For cyclic I/O connections, a valid 

ExpPacketRate

 is required for 

your call to 

ncOpenDnetIO

. For COS I/O connections, a nonzero 

ExpPacketRate

 is recommended for your call to 

ncOpenDnetIO

 but 

can be set to a large value.

COS I/O

Back to Back

COS I/O Data

Changing Frequently

0 ms

5 ms

10 ms

20 ms

15 ms

Some of the 

Other I/O May 

Have Timed Out