Johnson Controls METASYS ASC, Networking And Troubleshooting Manual

Page: 29 / 48

Introduction—ASC and N2 Bus Networking and Troubleshooting Guide 29
6. Connect channel B to the N2– wire and connect channel A to the N2+
wire. (Do not use the oscilloscope ground lead.)
7. Compare the signal you measure with the waveform in Figure 15.
They should resemble the figure. A signal outside this range indicates
that the N2 Bus is not functioning properly.
Note: At the dead time between messages, you can measure the bias
voltage. In this figure, it is 0.5 VDC.
Factors that can cause improper signal levels are:
●
N2 Bus wires are not polarized correctly (i.e., wires are reversed)
●
N2 cable runs are too long
●
breaks in the 24 AWG wire
●
an ASC has malfunctioned
●
N2 Bus is improperly terminated (e.g., EOL jumpers configured
improperly)
●
N2 Bus contains too many devices, loading down the bus
●
N2 Bus wiring has “T” or “Y” connections installed
Check all wiring and devices to pinpoint the problem. Replace parts if
necessary.
The AHU102-0 has two diagnostic LEDs that indicate whether the
controller is communicating over the N2 Bus and Zone Bus. During
normal operation, the two LEDs are used as Table 6 shows:
Table 6: Diagnostic LEDs on AHU102-0 Controller
Top LED
(N2 Bus)
Single blink The AHU is receiving but has not transmitted over the
N2 Bus during the past 1.5 seconds.
Double blink The AHU is receiving and transmitting data over the N2
Bus.
Bottom LED
(Zone Bus)
Single blink The AHU is transmitting but not receiving over the
Zone Bus.
Double blink The AHU is transmitting and receiving data over the
Zone Bus.
Top and
Bottom LEDs
Together
Double blink
together The AHU102-0 is resetting. An AI point with a value
greater than 10 VDC is most likely causing this
problem. Check current inputs.
Both off Power to the AHU is not on. Check the green LED on
the AHU101 motherboard.
Diagnostic
LEDs on the
AHU