Rockwell Automation 1761-NET-DNI DeviceNet Interface (DNI) - Manual (Page 74)

Publication 1761-UM005B-EN-P - October 2006
74 Application Examples
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
the DeviceNet network. 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.