Table 12: Guidelines for BACnet protocol MS/TP network topology
Category Rules and Maximums
Terminations Two FC devices with EOL switches in the ON position, one at each end of each FC Bus
segment
Note: Refer to the MS/TP Communications Bus Terminal Bulletin (LIT-12011034) or the Installation Quick Reference
Handbook (FAN410) for information on cable types and lengths.
Table 13: Guidelines for N2 network topology
Category Rules and Maximums
Two N2 Bus trunks supported.General
Only daisy-chained devices (with maximum stub length of 3 m [10 ft] to any device)
Number of Devices Up to 100 N2 devices supported per bus, with no more than two repeaters between network
engine and any N2 device and a maximum of 50 devices between repeaters
1,500 m (5,000 ft) twisted pair cable without a repeater
4,500 m (15,000 ft) twisted pair cable from NAE55 and the farthest N2 device (three segments of
1,500 m [5,000 ft] each, separated by repeaters)
Line Length and
Type
2,000 m (6,600 ft) between two fiber modems
Cable Solid or stranded 1.0 mm (18 AWG) 3-wire is recommended
Solid or stranded 0.5 mm (24 AWG) larger 3-wire or 4-wire (two twisted-pairs) is acceptable
Note: Use only twisted pair wire. On applications using 4-wire (two twisted-pairs) cable, iso-
late and insulate unused conductor.
Terminations Preferred Termination Configuration: Two N2 devices with EOL switches in the ON position, one
at each end of each N2 Bus segment
Minimally Required Termination Configuration: At least one N2 device with an EOL switch in
the ON position somewhere on each N2 Bus segment
Table 14: Modbus RTU rules
Category Rules and Maximums
General One direct Modbus RTU (RS-232) connection to either RS232C port is supported; connection
requires an RS-232/RS-485 converter to the RS232C serial port
One direct Modbus RTU (RS-485) connection to either FC Bus port is supported
Only daisy-chained devices
Number of Devices RS232C port supports one Modbus RTU (RS-232) device.
RS232C port with RS-232/RS-485 converter supports 32 Modbus (RS-485) devices or more,
depending on the converter used
Note: The typical maximum Modbus device count between repeaters is 32. Supported
Modbus RTU serial device count may be limited by manufacturer, cable, and communi-
cation speed. Check with the manufacturer for more information.
Cable Length RS-232 cable length can be up to 15 m (49.2 ft)
RS-485 cable length can be up to 1,520 m (4,987 ft)
Cable RS-232 stranded cable, 3-9 conductors, serial data grade, 20–24 AWG
RS-485 stranded cable, 0.6 mm (22 AWG) 3-wire twisted, shielded cable is acceptable.
Standard wiring qualified for use with the N2 Bus or FC Bus is acceptable
Stranded 0.6 (22 AWG) 4-wire (two twisted pairs) shielded is acceptable
Terminations RS-232: No termination
RS-485: End-of-line (EOL) termination must be set to On (or an EOL terminator installed)
on the two devices located at either end of each bus segment on an RS-485 bus. The EOL
switches must be set to Off (or EOL termination disabled) for all other devices on the bus
segment on an RS-485 bus.
NAE55 Installation Guide 13
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