Chapter 8: DeviceNet 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 8-5. Maximum Current on a Drop Line Relative to its Length For the calculation of the maximum current at a specific length, use the following formula: Where:
l = Length of the drop line in meters (m) i = Maximum current in amps This calculation applies to the sum of the currents of all the nodes on the selected drop line. The length (l) is not the cumulative length of the drop line; it is the maximum distance from any node on the drop line to the trunk line. It is important to note that voltage differences between the V– and V+ conductors need to be between 11 V and 25 V. The common-mode voltage between any two places on the V– wire must not exceed 5 V. SmartController EX User’s Guide, 11069-000 Rev. F Page 89 of 92 Length of Drop Line Maximum Current 1.5 m (5 ft) 3 A 2.0 m (6.6 ft) 2 A 3.3 m (10 ft) 1 1.5 A 4.5 m (15 ft) 1 A 6 m (20 ft) 0.75 A