Johnson Controls METASYS ASC, Networking And Troubleshooting Manual

Page: 25 / 48

Introduction—ASC and N2 Bus Networking and Troubleshooting Guide 25
This test applies only to the VAV100/101 and UNT100/101 Controllers
only. You’ll need a 100K ohm resistor and a DMM to perform this test.
Follow these steps:
1. If you have a Companion PC Version, remove the 25-pin RS-232
connection from the MM-CVT101 converter. This removes the
fluctuating voltage due to communications and allows you to read the
DC bias voltage. Plug the converter into a 120 VAC source.
If you have a Companion Panel/LTD, connect the Panel to a 24 VAC
source.
2. Connect a 100K ohm 1/4-watt resistor across the DMM. (For easier
installation, insert a double banana plug into DMM.)
3. If you cannot guess which ASC may be causing the ground loop,
follow Steps 3a and 3b below.
If you suspect which ASC is causing the ground loop, follow Step 3c
on the next page.
a. Connect one DMM lead to REF, N2+, or N2– and the other to
earth ground (Figure 12). Read the VDC/VAC scale on the DMM.
If the value is greater than 1 VDC/VAC, a ground loop is present
and therefore the N2 Bus is not properly isolated. Subdivide the
N2 Bus and measure again. Continue to subdivide the bus until
you locate the source of the problem. Correct the isolation
problem and measure again.
If the value is less than 1 VDC/VAC, the N2 Bus is properly
isolated. A value less than 0.10 VDC/VAC indicates a completely
isolated circuit.
Figure 12: Measuring N2 Bus Wires to Earth Ground
TEST 1: Earth
Grounded 24
VAC Load
UNT100/101 or any VAV
KOHMRST
If DMM < 1 VDC/VAC = OK (Isolated)
If DMM > 1 VDC/VAC = Ground Loop
100K ohm
1/4-watt
N2+
N2-
REF
Companion Panel/
LTD or MM-CVT101*
DMM
N2+
N2-
REF
* RS-232 cable disconnected.
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