69BDeviceNet Messaging Protocol
7BChapter Four: Operation
46
Note
The DeviceNet messaging protocol is based on hexadecimal (base 16) math, rather
than decimal (base 10) math. Descriptions in this chapter and the examples in
Appendix D: Example Messages
, page 111, assume a basic understanding of
hexadecimal math.
In the actual application of DeviceNet communications, the messaging process typically is
automated using an interface software program; therefore, manual calculation and interpretation
of the messages is not required. Typical interface programs require only the input of specific class
codes, instance IDs, attribute IDs, and any required data variable(s). Refer to the ODVA
“DeviceNet Specification,” Volume I and Volume II [1, 2] for a complete description of the
DeviceNet messaging protocol.
Message Formats
Explicit Messaging Connections
Explicit messaging connections utilize a direct request/response format that enables you to access
any attribute data. Explicit messaging is typically used for device setup, configuration, and
calibration.
Explicit messages
⎯
both requests and responses
⎯
consist of two segments:
•
CAN Identifier Field
, which defines the type of connection.
•
CAN Data Field
, which defines the particular request or response.
I/O Poll Messaging Connections
I/O poll messaging connections utilize an assembly format to group and report data from multiple
objects using a single communications command. These connections typically are used for quick
reporting of information (run-time). Although I/O poll messaging connections typically report
data faster than explicit messaging connections, they limit your access to the set of attributes (the
exception status and the pressure value) listed in Tables 32 and 33, page 64. The data format in
I/O poll messaging connections is pre-defined and cannot be altered.
I/O poll
request
messages consist of a single segment:
•
CAN Identifier Field
, which defines the type of connection.
I/O poll
response
messages consist of two segments:
•
CAN Identifier Field
, which defines the type of connection.
•
Poll Response
, which reports a fixed set of attribute data defined by the Predefined
Master/Slave Connection Set [1].
There are two types of poll responses, each reporting a different set of data. The poll response
that is returned is defined in the Device Configuration Object (page 71). The actual data for either
poll response is stored in the Assembly Object (page 58).
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