AHRS using backup RS232

[stautz]

I get a message after I start on my G3X that the AHRS is using the backup RS232 connection.

Is it supposed to run off the CAM bus on pins 45 & 46 of connector P4602? If so, wouldn't my other CAM bus devices not be working?

Any help is appreciated.

[Pete D]

Hi Steve, I ran this by our avionics team and they provided the following troubleshooting suggestions:

There is a problem with the CANBUS at the ADAHRS. Pin 1 on the ADAHRS should ring to pin 46 at the GDU, and pin 2 should ring to pin 45.

Assuming an FX2/3, if there is no continuity, the first place I would look is at the wing root connector P/J 113…we have seen these separate just a fraction and cause loss of continuity on the CANBUS while everything else works. CANBUS runs through pins 1 and 2 there.

[stautz]

I checked the continuity from pin 1 on the ADAHRS to pin 46 at the GDU, and pin 2 to pin 45 and both had continuity. I didn't know where to go from there, so I put everything back together and tested it. I did not get the message on start-up, so I'll see if it repeats in the future. I am guessing that it was the ADAHRS connector that had the problem.

[Andy]

In a standard CubCrafters G3X installation the GSU 25 is the last LRU on CANbus and must be fitted with a CANbus terminator.

Is there a terminator (small male/female connector block) between your harness connector and the GSU 25 connector?

CANbus may work without the terminator but it will not be reliable.

[stautz]

I have the G3X Touch panel with the G5. I did not see a CANbus terminator at the GSU25. I'll have to look to see where it might be installed if not there.

[Andy]

The standard CubCrafters addition of a G5 to CANbus does not change the position of the GSU 25 on CANbus. The standard installation opens the CANbus at the GDU to insert the G5 between the GDU and the GEA 24.

The GSU 25 must be fitted with a terminator.

[Andy]

The standard CubCrafters addition of a G5 to CANbus does not change the position of the GSU 25 on CANbus. The standard installation opens the CANbus at the GDU to insert the G5 between the GDU and the GEA 24.

The above applied to G3X Touch aircraft that had the G5 added by the mod center after they came off the production line. I assume that CubCrafters now installs the appropriate wiring harness during factory build and includes the appropriate harness in a kit deliveries.

I have no documentation on any revised harness and don't know if the G5 is in the same CANbus position as was used in the earlier mod.

Bottom line is that both ends of CANbus must be terminated. On my aircraft one end is the GSU 25 and it has the separate terminator. The other end is the GSA 28 pitch servo. It does not need the separate terminator as it has an internal 120 ohm resistor that is put in circuit by linking pins 3 and 4.

[stautz]

I checked and the CANbus terminator is installed at the GSU25. My is wired like yours with the other end terminated at the GSA28.

[Andy]

I checked and the CANbus terminator is installed at the GSU25.

That would have been a lot easier to fix than finding an intermittent wiring or connector problem. I don't know how long it takes for GDU to revert to serial after loss of CAN data. The GSU 25 status LED may give a faster indication if you can have someone watch for changes as you flex the wiring harness or wing root connector.

No Light > No Power
Steady Green > On, but not communicating using the CAN Bus
Flashing Green > On and communicating using the CAN Bus
Red > Hardware Fault
Alternating Red/Green > CAN bus network error two similar devices are configured with the same unit ID