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6.4 DeviceNet Physical Layer and Media
to operate all the attached nodes. This must comply with national and international
safety standards.
Make sure that each power supply incorporates current-limit protection.
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Make sure each power supply is temperature-compensated.
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Provide over-current protection for each segment of your DeviceNet cable installation.
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The SmartController EX CAN driver (CAN_H / CAN_L) is protected for shorts to the power ter-
minals. The driver is protected for voltages in the range from -27 to +40 V.
By default, the 24 V supply for the DeviceNet bus on the SmartController is disabled.
If you do want the controller to supply 24 V, two jumpers need to be moved on JP1, on the
SmartController board. To do this, move the jumpers from the EXT position to the INT pos-
ition. See Figure 6-5.
The jumpers provide an internal source of 24 V and GND from the EX controller for the
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CAN+CAN+ / CAN-- lines.
Power is polyfuse-protected and can source 24 V (equal to controller V
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Power is diode-protected so it cannot back-drive the controller power.
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Power Capabilities of a DeviceNet Cable System
A DeviceNet cable system has several power rating constraints. The cable type and the length
of the cable affect the maximum current on a cable. Thick and thin cable have:
24 VDC power rating
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Optional power-supply tabs
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If the power supplies are equipped with Schottky diodes, the optional power supply tabs must
be protected from bus back-feeding of current among the power supplies on the bus. We also
recommend the use of fuse protection for every trunk line in the cable system.
The maximum current rating of a thick cable trunk line is 8 A. Verify that this complies with
your national and international standards. It might be necessary to limit the maximum current
to a lower value if standards in the U.S. or Canada apply. The maximum current value is a
theoretical value. The cable size supports a higher current than 8 A. Depending on the topo-
logy of nodes relative to the power supply, higher currents might be possible. See the
DeviceNet technical specifications for further information.
The maximum current rating of a thin cable trunk line is 3 A. If you use the thin cable on a
long line, the resistance of the cable decreases the maximum current value. See the following
table and the DeviceNet technical specifications for further information.
Table 6-8. Maximum Current on a Drop Line Relative to its Length
Length of Drop Line
1.5 m (5 ft)
2.0 m (6.6 ft)
3.3 m (10 ft)
68
Maximum Current
3 A
2 A
1
1.5 A
SmartController EX User's Guide
) at up to 1.0 A.
IN
11069-000 Rev. H
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