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Issue
99, October 1998
Networking
with DeviceNet—Part
2: A Weather Station Application
by
Jim Brady
Start
• Can
Chips
• Chip
Setup •
Real Time
• Message
Flow
• Connections
• Timers
• Analog
Input Point
• Identity
Object
• Fragmented
Messages
• Getting
Physical
• Applying
DeviceNET
• Software,Sources
MESSAGE
FLOW
Figure
1 shows message routes in the system. Explicit and
I/O Poll messages come in through their respective mailboxes.
Explicit messages are routed via the path specified in
the message and can access almost any object in the device.
I/O Poll messages grab preselected data from a buffer
in the Assembly object and quickly send it.
The
weather-station sends three bytes—device status,
temperature, and humidity. I can send more data by adding
it to the existing assembly or creating a second assembly.
The device manufacturer determines which data goes into
the assemblies.
At
the top of Figure 1 is the unconnected port, which the
master uses to allocate the connections it wants to use.
Technically, connections don’t exist prior to allocation.
This
situation implies using C++ dynamic allocation. Although
you can do this, I chose to create static objects at the
beginning of main() and use the constructor to set the
initial connection state to nonexistent.
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