Rockwell Automation 198x Modular DeviceNet Starter Auxiliary I/O - Manual (Page 47)

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Publication 198-UM001B-EN-P September 2001
Modular DSA system using explicit messaging. Explicit messages contain the following
information:
• Service – Tells the device what action to take in response to the message. Service
reads information from a device and for writes information to a device.
• Class – Tells the device which object class to send the service to. Classes are
identified by their numeric “class code”. Appendix A contains a complete list of the
classes that are implemented in the Modular DSA system.
• Instance – Each object class can contain one or more “instances” of that class in a
given device. Instances are numbered starting with instance 1. The value ”0” refers to
the class itself, not any individual instance of that class. Appendix A contains a
complete list of instances that are implemented for each class in the Modular DSA
system.
• Attribute – An attribute is a single piece of information related to an object class or
instance. Attributes are numbered starting at attribute number 1. Note that attributes
need to be numbered sequentially. Appendix A contains a complete list of attributes
for each class and instances implemented in the Modular DSA system.
All attributes can be read using the ”Get Single Attribute” service to that particular attribute.
Some attributes can be written to using the “Set Single Attribute” service to that particular
attribute. There are a few special rules to keep in mind when trying to control an output with
explicit messaging.
• An output cannot be controlled (Set Single Attribute service) via explicit messing if a
DeviceNet I/O connection exists. For example, if a DeviceNet scanner has control of
a Modular DSA system, then an explicit message attempting to control an output will
result in the Modular DSA returning an error response of “Object State Conflict”,and
the output will not change state.
• An output cannot be controlled via explicit messaging unless the explicit connection
has a non-zero Expected Packet Rate. (See DeviceNet Spec. Vol1 for details on the
explicit message connection.)
• If the explicit connection is released/closed, all outputs that this connection has
controlled will go to the IDLE STATE.
• If the explicit connection times out, outputs that this connection has controlled will
go to the FAULTED STATE.
DeviceNet I/O Messaging
DeviceNet I/O messages are used by Allen-Bradley DeviceNet scanners to turn on the
Modular DSA outputs, to read the status of the I/O system inputs, and also to read the status
fault information for the system. The size and format of the DevicNet I/O messages is
determined at power-up. After determining what type of modules are attached and whether
they are in the same configuration as that stored in the EEprom, the scanner will build I/O
messages.