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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
FRAGMENTED
MESSAGES
The
weather station’s I/O Poll response is just three
bytes—one byte each for device status, temperature,
and humidity.
If
I used floating point or added more sensors, the CAN message
limit of 8 bytes would quickly be exceeded. I’d need
to send the data in two or more fragments.
I/O
message fragments are like normal messages except the
first byte provides a fragment flag and a fragment count.
That leaves seven bytes for data.
For
maximum speed, I/O message fragments are sent back-to-back
with no acknowledge (ack) message from the master other
than the CAN level acknowledge bit.
Fragmenting
an Explicit message is more complex. You send a fragment,
wait for an ack message, and send the next fragment. If
the ack takes too long, resend the fragment. If you time
out again, give up trying to send the message.
Many
error cases can arise, like getting an ack from the master
with a fragment number different from what you sent, getting
a message that’s not an ack while you’re sending
fragments, and so on.
The
weather station is capable of sending and receiving fragmented
Explicit messages. Its serial number and product name
are long enough to require it.
Fragmented
messages are a big part of DeviceNet conformance testing.
My program managed to pass a self-inflicted test using
the ODVA conformance software. This software generates
every conceivable bogus response and breaks all but the
best code. You like challenges, right?
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