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DeviceNet
Introduction |
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DeviceNet™
Unplugged
A View “Under the Hood” for End Users |
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| DEVICENET AND CAN |
Controller Area Networking or CAN is
a communications standard with a rather prolific set of
offspring that includes DeviceNet, Can Open, Can Kingdom
and several hundred other offspring all over the world.
Controller Area Networking is a serial communications
standard for intelligent devices to communicate with each
other. Unlike many other communication standards that
provide fast data rates with thousands or millions of
data bytes in a single frame, CAN has a bit rate that
max's out at 1 mega baud. Most industrial applications
don’t even need that speed. Most use the lowly 125Kbaud.
And where other standards move thousands of bytes in a
single frame CAN only moves 8 bytes of data.
But where speed and capacity are strengths for many of
the other standards, CAN’s strength is its low overhead
and simple physical interface. With its small packet size,
even at 500Kbaud a frame with eight bytes of data is only
on the network wire for a quarter of a millisecond. For
many control applications this is plenty fast.
Additionally the microcontroller, yes an underpowered
8-bit micro can get the job done, needs as little as 4K
of program memory and 256 bytes of RAM to support a CAN
application.
CAN was created by Bosch in Germany in March of 1985.
The Bosch Company designed it to replace automotive wiring.
The Bosch Company designed it to replace automotive wiring.
In the early days of specification version 1.2, CAN messages
contained an eleven-bit identifier providing the capability
to address 2047 identifiers. In 1992 CAN Specification
2.0 extended the identifier size to 29 bits providing
up to 56 million unique identifiers. As both specifications
are still in use (sometimes on the same wire) the original
1.2 specification is called Part A and the new specification
2.0 is termed part B. A unique attribute of CAN is that
only two of the OSI Reference Model layers are defined
(Figure 1), the
Data Link Layer and the Physical Layer. The CAN Data Linker
is normally split into two sub layers, the Physical Signaling
sub-layer and the Media Access Control (MAC) sub-layer.
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