1761-6.5
Application Examples
7-7
DNI - COS (change of state) I/O Messaging
The ladder logic shown below is an example of how to send input data to the
DNI (COS input messaging). This procedure is recommended because the
latency issue associated with polling devices is minimized. Polling is a
mechanism that is used to acquire data from a device. The DNI reading data
from a controller at a set interval is an example of a polled relationship. The
problem with polling is that usually the data does not change, so the
communications are consumed by reading the same data over and over.
Another problem is that information cannot be received quicker than the
polled rate, and depending on when the data changes, worst case update can
take up to twice the polled rate.
One solution to this problem is to allow for COS (change of state)
messaging. What this allows is for the device (MicroLogix for example) to
send input data to the DNI when it detects new/changed data. Many also call
this type of relationship unsolicited communications. This can significantly
improve system throughput because the polling interval is eliminated.
The way the MicroLogix controllers can get input data into the DNI is to
message to node address 254. Address 254 is an invalid address on
DeviceNet. When the DNI detects an incoming message, it looks at the
destination address and size; if it reads 254 for the address and the size is
equal to the configured input size, it knows the data in the message is input
data destined for the DNI (For DNI Series B, the size includes master and
explicit input data). The data is then placed into the DNI’s input image and
is delivered to the DeviceNet master using the polled or COS connection
that was configured in the DeviceNet master by the user.
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