8 IM 730-0
N2 Bus Rules
Figure 6 summarizes the rules and maximums allowed for installing the N2 Bus. You may wish to
print this table and keep it handy.
Figure 6. N2 Bus Rules
Category Rules/Maximums Allowed
General
One or two N2 Bus per NCM
Only daisy-chained devices
Number of Devices
100 devices per NCM (60 to 200 TC-9100s)
50 devices per repeater
Two repeaters cascaded
Line Length and Type
1524 m (5000 ft) between NCM to farthest N2 device before repeater is needed
4572 m (15,000 ft) from NCM to farthest N2 device (three segments of 1524 m [5000 ft]
each)
2012 m (6600 ft) between two fiber modems
Cable
26 AWG twisted pair or larger
(solid or stranded 22 AWG or heavier recommended)
Terminations
Two switched EOL per segment (preferred)
One switched EOL per segment (required)
Number of Devices
• Currently, up to 100 devices can be connected to an NCM, including repeaters. The actual
number of devices is dependent on the features and point count used in the NCM.
Note: The number of devices varies because both the number of software objects and JC-BASIC
processes influence the Network Controller’s (NC’s) performance (see Guidelines for
Efficient Operation Technical Bulletin [LIT-636341]).
• Up to 50 daisy-chained devices are allowed before a repeater is needed. Add a repeater to the
bus when you reach 49 devices. Count each repeater as one device.
• Any path from the NCM to an N2 device cannot go through more than two repeaters or two
pairs of fiber modems (i.e., cascaded repeaters/modems). This is because the repeater/modem
delays the N2 Bus signal between Sides A and B. The N2 Bus can compensate for only two of
these delays; therefore, up to two repeaters or two pairs of fiber modems can be cascaded in
series. Note that the signal from the NCM only passes through two repeaters or two pairs of
modems to any N2 device.
• For additional information on the maximum number of devices and priority assignments, be sure
to read Guidelines for Efficient Operation Technical Bulletin (LIT-636341).
Line Length and Type
• A repeater is required for every 1524 m (5000 ft) of daisy-chained cable.
• The maximum distance between an NCM and the farthest device (even through repeaters) is
4572 m (15,000 ft).
• You may use 18 through 26 AWG twisted pair wire; however, Johnson Controls recommends
22 AWG or heavier, because lighter wire breaks easily when stripped and installed.
• The maximum distance between two fiber modems is 2,012 m (6,600 ft). If your application
requires lengths beyond that, contact S.I. Tech for information about their “high power” option.
You may also use optical fiber on the N2 Bus (a pair of fiber modems is required for conversion).
Duplex optical fiber is needed, either 50 (3.0 dB/km), 62.5 (4.0 dB/km), or 100 (5.0 dB/km)
micrometers. The 62.5 size is preferred.
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