National Instruments NI-DNET User Manual
Page 57
Appendix A
DeviceNet Overview
A-10
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The cyclic I/O connection enables a slave device to send input data to its
master at the configured EPR interval. You normally configure the EPR to
be consistent with the rate at which the device measures its physical input
sensors. For example, if a temperature sensor can take a measurement at
most once every 500 ms, you would configure the cyclic I/O connection’s
EPR as 500 ms. Cyclic I/O can be configured to send output data from
master to slave, but this configuration is seldom used since it is essentially
the same as polled I/O. Cyclic I/O messages can contain any amount
of data.
The COS I/O connection enables a slave device to send input data to its
master when a change is detected on its physical inputs. In addition to
sending input data when a change is detected, the COS slave also sends its
input data at a slower EPR interval that lets the master know it is still
functioning. COS I/O is typically used for devices with physical inputs that
can change frequently but can have the same input value for a long time.
For example, if a pushbutton device supports COS I/O, you might
configure its EPR as 3 seconds since the device sends a message
immediately if a button is pressed. COS I/O can be configured to send
output data from master to slave. Although master-to-slave COS output is
seldom used, it can be useful for things like front-panel pushbuttons which
are sent to a slave’s discrete outputs (such as LEDs and simple motors).
COS I/O messages can contain any amount of data.
When using COS/cyclic I/O connections, you can configure the device that
receives data to send an acknowledgment so that the transmitting device
can verify that the data was received successfully. For example, if you
configure slave-to-master COS I/O (input length nonzero), the master
sends an acknowledgment to the slave each time it receives an input
message. Since the acknowledgment message is used for verification only,
it does not contain data. If this verification can be handled using other
means (such as using strobed I/O to verify device status), the
acknowledgment message can be suppressed. For information on how to
suppress COS/cyclic acknowledgments using NI-DNET, refer to the
description of the I/O Object in the NI-DNET Programmer Reference
Manual.
Since COS and cyclic I/O use the same messages on the DeviceNet
network, they cannot be used simultaneously for a given slave device.
Also, polled I/O uses the same messages on the DeviceNet network as
master-to-slave output messages of COS/cyclic I/O. This means that a slave
device can use slave-to-master COS/cyclic I/O simultaneously with
polled I/O, but not master-to-slave COS/cyclic I/O.