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Issue
98, September 1998
Networking
with DeviceNet
by
Jim Brady
Start
• Sorting Them Out
• New
Breed •
Motivation •
Can •
Message Reliability •
DeviceNet Connections •
Device Net Messages •
Stringing Messages Together •
Some Real Messages •
Object Library •
Conformance Testing •
DeviceNet Standards •
References,Sources,PDF
Stringing
Messages Together
An
explicit message from the master uses five bytes of the
eight bytes available for the path, service, and master
MAC ID. This leaves only three bytes for actual data.
If
the master sends a four-byte long int, two separate messages
are needed. These messages are called fragments and are
just like a regular message, except the first part of
the data field contains fragment information.
For
an explicit-message fragment, the first byte contains
a fragment flag, and the second byte specifies the fragment
type (first, middle, last) and the fragment count. The
fragment count is only a six-bit value but can roll over
any number of times, allowing for messages of unlimited
length.
For
an I/O-message fragment, only one byte is used for fragment
information (the one specifying fragment type and count)
to maximize the space for actual data. In this case, the
fragment flag is implied if the produced connection size
is greater than 8.
For
explicit or I/O fragments, the receiver simply concatenates
the data obtained from each fragment, stopping when it
sees the flag indicating the final fragment.
With
fragmented messages there is the question of whether the
receiving device needs to send an acknowledge for each
fragment received. It does for an explicit message but
not for an I/O message. I/O message fragments are sent
back-to-back for maximum speed.
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