Decoding Metasys N2 Messages: Cable Tips for JCI

This section describes briefly what you need to know when designing an N2 Bus installation.

A properly installed N2 Bus consists of an electrically continuous line of cable (unbroken) from one end of the system to the other.

For most N2 Bus installations, the most practical choice is solid, two twisted pair unshielded telephone cables. If you have existing stranded cable, you can use it, but you may find that the strands become a nuisance when wiring the cable. However, stranded wire is stronger than solid wire, since it is not as prone to breakage during installation.

For N2 Bus installations where the very high noise exists (e.g., gas ignition systems, radar or magnetic resonance imaging equipment, factory floor, or outdoors), shielded wire or optical fiber is best. Of the two, fiber is by far the better, but the more expensive. It offers extended N2 distances and excellent immunity to electrical noise, lightning, and various other building noises. It may be buried underground between two buildings, so that the N2 Bus can be extended in a campus-type installation.

Table 2 summarizes the rules and maximums allowed for installing the N2 Bus. You may wish to photocopy this table and keep it handy. These statements are explained in following sections.

Table 2: 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 5124 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) Design Considerations Selecting the Right Cable N2 Bus Rules 12 Network Communications—N2 Communications Bus General

• Each NCM can support one or two N2 Bus. The devices that you place on the N2 Bus must be daisy-chained devices, which include: NCM, DCM, XBN, XRE, XRL, XRM, IAC, ILC, AHU, UNT, VAV, VMA, LCP, IFC, D600, repeater, and surge protector. For more information on using two N2 Buses, refer to Dual N2 Bus Application Note (LIT-6363145) under Application Notes: Network Communications in this manual. 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.
• Special rules apply to the TC-9100 controller, where the maximum number of devices can be from 60 to 200, dependent on the software configuration in the NCM.
• 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. These configurations are shown in Figures 12, 13, and 14. 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) under the Appendix tab of this manual. Line Length and Type
• A maximum of 1524 m (5000 ft) of daisy-chained cable is allowed before a repeater is needed. Add a repeater to the bus when you reach 1524 m (5000 ft).
• The maximum distance from an NCM to 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. Network Communications—N2 Communications Bus 13
• The maximum distance between two fiber modems is 2012 m (6600 ft). If your application requires lengths beyond 2012 m (6600 ft), 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.