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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
GETTING
PHYSICAL
DeviceNet
requires you to keep the network data and power isolated
from green-wire ground by 1 MW or greater. If anything
can reference your circuit to green-wire ground (e.g.,
an RS-232 port), you must optoisolate the network.
My
PC/104 DeviceNet interface is shown in Figure 3. The weather station is isolated from ground and
has no ports other than DeviceNet, so I didn’t need
optoisolators.
Power
consumption is 5 W, so I powered the whole thing from
DeviceNet power. The voltage varies between 11 and 25
VDC, so use a wide input-range DC-to-DC converter.
DeviceNet
also needs a miswiring-protection circuit, which lets
you mix up the network connections in any possible way
without frying your device or the network. The DeviceNet
specification includes a circuit for this. The Philips
82C251 CAN transceiver has ESD protection and line protection
up to 40 V continuous.
DeviceNet
is fairly specific in its interface guidelines. I used
two BCD rotary switches to set MAC ID and one more for
data rate. I also went with the recommended bicolor LED
for module and network status.
Of
the three network-connector choices, I used the circular
micro style. It has five pins—two for differential
data, two for power, and one for the drain wire.
The
data lines are referenced to power V–, so your CAN
transceiver must also be referenced to this to prevent
exceeding its common-mode voltage range.
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