Johnson Controls METASYS ASC, Networking And Troubleshooting Manual

Page: 27 / 48

Introduction—ASC and N2 Bus Networking and Troubleshooting Guide 27
You’ll need a DMM to perform this test. By connecting the DMM to each
N2 Bus wire, you’ll be able to detect polarity, shorts, crossed wires, and
grounds.
Follow these steps, referring to Figure 14:
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 in the converter’s transformer into a 120 VAC
source.
If you have a Companion Panel/LTD Version, make sure the
Panel/LTD is powered with 24 VAC. Note that the voltage reading on
the DMM may fluctuate, since the bus will be communicating.
2. Connect the DMM across the N2+ and REF screws on the N2
terminal block (Figure 14). Write down the DC voltage reading.
Repeat for N2– and REF, then for N2+ and N2–. Compare the
voltages you read on the DMM to the following values:
N2+ to REF = +2.45 to 2.98 VDC
N2– to REF = +2.06 to 2.54 VDC
N2+ to N2– = +0.36 to 0.92 VDC
If your readings are not approximately within the ranges listed above,
that particular wire is grounded, shorted, or crossed with another wire.
Correct and measure again.
If your readings are approximately within the ranges listed above, the
bus is properly wired.
Figure 14: Test for N2 Bus Shorts, Crossed Wires, Grounds
TEST 3: N2 Bus
Polarity, Shorts,
Crossed Wires,
Grounds
POLARTY
N2+ to REF
+2.45 to 2.98 VDC
N2+ to N2-
+0.36 to 0.92 VDC
N2- to REF
+2.06 to 2.54 VDC
GND
N2 Bus Terminal Block on
MM-CVT101 or Panel/ LTD Companion
DMM
N2 -
N2 +
REF
N2 Bus Terminal Block
DMM
N2 +
N2 -
REF
For best reading, place probe on metal
plate inside terminal, not on screw. Note:
DMM
N2 -
N2 +
REF
ZBus
UNT / VAV
AHU / LCP