Garmin G3X - Page 53

190-01115-01 G3X™/G3X Touch™ Avionics Installation Manual
Rev. AV Page 2-29
2. Make sure the CAN bus is daisy-chained between CAN device around the system, and that CAN
devices are not connected by a single point like a star or routed through a hub device. This can
cause unwanted signal reflections and “orphan” some devices on the bus and prevent their
communication.
3. Make sure the CAN bus is terminated in only two locations, and only at the extreme ends of the
CAN bus.
4. With power removed, remove a connector from one of the devices that is not
located at either of
the extreme ends of the CAN bus.
a) Using an ohm meter, verify the resistance between the CAN HI and CAN LO pins on the connec-
tor is 60 ohms. This will verify the CAN backbone is properly terminated at each end (two 120
ohm terminating resistors in parallel).
b) A resistance of 120 ohms indicates that one of the two required CAN terminations is missing.
c) A resistance of 40 ohms or less indicates that too many terminations are installed.
5. Verify the CAN HI and CAN LO signals are not swapped, shorted together, or open-circuited at
any LRU connector.
6. Verify the CAN HI and CAN LO signals are not shorted to ground (this can happen when shielded
wire is installed incorrectly).
7. Highlight each device on the configuration mode system information page (Section 30.4.3
) and
verify the value displayed for Network Error Rate is a steady 0% (Figure 2-16).
Figure 2-16 Network Error Rate (configuration mode)
8. Start only the PFD1 display and one other CAN device at a time, and verify the connection quality
for each device. Sometimes a device will communicate with PFD1 only when it is the only
powered device on the CAN bus, if one or more of the above issues is present. Evaluating each
CAN device in turn can help narrow down a problem.
9. It is very important for each device on the CAN bus to share a common power/signal ground.
Ground potential differences between devices on the CAN bus can cause communication errors.
Ground devices to a common ground bus, not to the airframe or to multiple grounding buses.