N2 communication offline

[westendmechanical]

I service building that has about 60 feild devices, unt's,vav-101 and DX9100,
Supervisery controller is a NCM 350. All of the network is offline, No work has been done in the building for a while. No floods no power failures, NOTHING! What DC Volts should I be looking for between com And(+) com and (-) on the N2 bus I was told it's adout 2.7 DC Volts. I need to know if there is a cut in the network. If there is a cut what should the volts be. I checked all N2 connections at all DX9100 because they are AHU'S and RTU'S. All connections are good. If I wanted to separate the N2 bus, how do I know which way the cut or bad connection is. Any help would be great.

[sarge7359]

How about hooking it up to a telephone style warbler? I don't recall those devices using any DC voltage, but you might want to double-check to be sure.

Telephone techs use them all the time to track down where the phone wire goes and if the wire has been cut.

[Cdoanth]

Check N2 bus by DX Commissioning or HVAC Pro first
Check ground fault on N2 Bus

[kevin stone]

I don't know if you have this doc but see if it helps
I find most of the time the bus is grounded it is pretty easy to find if you are
familiar with a o scope
Kevin

[The Reaper]

Is the NCM online to the head end?

[steves4]

There are multiple discussions in this forum about diagnosing problems with the N2 bus. Most of them were discussing problems with one, or a few, devices whereas your entire bus is offline. However, the same tecniques will work to isolate and repair your problem. Here is a summary of the guidelines for the N2 bus that I've gathered from various JCI sources.

References (available at http://www.johnsoncontrols.com):
Johnson Controls, Inc. ASC and N2 Bus Networking and Troubleshooting Guide LIT-6363003
Johnson Controls, Inc. N2 Communications Bus LIT-636018
Johnson Controls, Inc. N30 Supervisory Controller Installation LIT-6891100

N2 Network Voltages
On a properly terminated and biased N2 network the following voltage levels should be observed on an idle network:
Voltage from N2+ to N2- = +0.36 to 0.92VDC* (DMM+ probe on N2+ terminal; DMM- probe on N2- terminal.
Voltage from N2+ to REF = +2.45 to 2.98VDC (DMM+ probe on N2+ terminal; DMM- probe on N2- terminal.
Voltage from N2- to REF = +2.06 to 2.54VDC (DMM+ probe on N2- terminal; DMM- probe on N2- terminal.
*If the difference from N2+ to N2- is greater than -.36V your system should be able to communicate.

EOL Termination
Many of the Johnson Controls Metasys ASCs and controllers include self termination logic within the controller itself. These include the AHU, LCP, UNT, and VAV.
Johnson Controls Metasys devices that include switch, or jumper, selectable termination include NCM, DCM, XBN, XRE, XRL, XRM, ILC, IFC, D600, and repeater.
For each N2 bus segment, at least one switch-terminating device must have its EOL set to In. This is typically the NCM or a repeater. However, the N2 bus will be less susceptible to noise if the ends of each segment have a switch-terminating device with its EOL set In.

[GONFISHN]

How far is it from the NCM to the fiest field device? If its not too far you can just string out some wire on the ground and disconnect the outgoing comm from the first module and hook up the wire you strung out only. You know this wire is not cut and should help establish your first comm link. Then you can go from there.

[westendmechanical]

Thanks to all that responded, What I found was a shorted N2 bus at a DX9100 on a rooftop unit. What I did was remove all N2 conections at a device and have someone sit in front of the OWS a see if it would clear. I did check this DX the day before and saw nothing wrong with any of the N2 bus connections. This was late Friday afternoon, now eveything is back online except for that rooftop. I will check Monday why that DX shorted the N2 bus.:)