ASCB BUS Troubleshooting 

For in depth ASCB BUS troubleshooting recommendations and test equipment detail, refer to the ASCB Fault Isolation Guide.

As part of the No Fault Found (NFF) reduction program for Data Acquisition Unit (DAU), the data analysis shows that two of the trends were communicated to operators and if the troubleshooting recommendations are correctly followed, it would prevent unnecessary removals of DAU, thus leading to substantial time and cost savings.

One reason for removal often seen as NFF is the difficulty or inability to download the DAU NVM. One should always remember that for the Honeywell IMT system that includes the DAU, the PMAT/CAIMS system provides detailed troubleshooting and corrective action with sufficient information to troubleshoot without the need for an NVM download. Therefore NVM download should only be restricted for remote cases under special request by Honeywell or Bombardier.

This approach will make the need to download the DAU NVM very infrequent, thus eliminating subsequent removals due to a download problem. In the event that the DAU NVM download is still required, and is specifically requested by a Honeywell or Bombardier representative, the download process should normally work without issues. However, if the download is not functioning correctly, then reset procedures of the appropriate DAU channels CB, or a restart of the PMAT and a retry of the download process will be required. This reset procedure is a known issue referenced in Advisory Wire AW700-45-0002 of the CAIMS Member System Functionality Status page 11 of 11, footnote 9. It reads as follows:

"If download is not functional then reset the appropriate DAU channel via CB and restart the PMAT; if the problem persists retry power down, wait 10 seconds and then power-up. That will prevent unnecessary removals of DAU and reduce the DAU NFF rate."

Another reason for removal often ending with NFF is 'DAU (1A, 1B, 2A,…4B) FAIL' CAS message that is posted when the corresponding DAU's NVM is full, and a corresponding DAU NVM FULL fault is being recorded in NVM. This is a known issue for the DAU part number 10604B01M05 only. When a DAU NVM is full and that DAU is attempting to record additional faults in its NVM, the resulting fault log processing would run longer, thus affecting the ASCB bus transmissions for the DAU fault registry. The flight deck effect of such scenario is a momentary 'DAU FAIL' CAS message that is posted while the fault is attempting to be logged in the NVM.

The most common scenario of this issue has been observed with the DAU when the flight crew initiated the Pilot Event Marker that also gets registered in the LRUs NVM just like any other fault that also resulted in a DAU Failure. Thereafter, upon landing, the ground crew downloaded the corresponding DAU NVM, and found that it was full. The issue will only occur on the flight where the NVM is full. Once the DAU goes through a cold-start on the ground after its NVM is full, the DAU will automatically clear its NVM. As a troubleshooting guide, if the 'DAU FAIL' advisory CAS message is posted intermittently or not, one should verify the DAU part number to be P/N 10604B01M05 and perform an NVM download on the corresponding DAU channel. If the NVM download is performed before the aircraft powers down when the issue appeared, and the NVM file is very large because it is full, then the issue can be attributed to this scenario. If the NVM download is performed after the aircraft power is cycled, and the NVM file is very small because it is already empty, then this can be attributed to the above described scenario. In either case, no further action is required by the customer since the NVM will be cleared anyway on the next DAU power-up when such problem occurs.

DU Interfaces

High Level Datalink Control (HDLC) – Each IAC has two Display Processors (DP’s) similar to having a computer with more than one video card installed. Each Display Processor has the processing capability to generate two displays. Each IAC Display Processor 1 is connected to all three left hand displays and each IAC Display Processor 2 is connected to all three right hand displays. 

Normal operation:

• IAC 1 DP 1 drives DUs 1 and 2. 
• IAC 3 DP 1 drives DU 3 and IAC 3 DP 2 drives DU 4
• IAC 2 DP 2 drives DUs 5 and 6.   

The Display Processors supply display formats digitally to the display units and a Symbol Generator (SG) within the display converts the digital data to the analog signals to drive the display.
The Symbol Generator in the displays supply ARINC 429 Wraparound back to all three Fault Warning Computers (feedback loop) to confirm what is on a display is as it should be/valid/what was originally generated by an IAC is correct. Wraparound is also used to detect Miscompares/Discrepancies between Pilot and Copilot displays.

Fault Reporting

A CHECK DU EICAS message is generated when the Display Unit feedback (Wraparound) does not match what was originally generated by the IAC. Following, are three possible display issues (shown graphically) that will generate a check DU message.

Three possible issues:

A Check DU message when you see a blank DU is indicative of a potential DU failure.

• The DU is not receiving power
• The DU is receiving power but failed (ie. internal cold solder joint etc)
• The DU is receiving a Power Down command from the Reversion Panel. When reversions (PFD, EICAS) are selected a power down command is sent to the original display. Using the schematic manual, find the power down command and confirm. If Ground is present, replace Reversion Controller or find the (faulty) ground.

Swapping a DU to another location is an easy way to confirm DU is failed or not. All DUs are interchangeable.

A CHECK DU message when you see a red X is indicative of a potential DU input failure. The Symbol Generator within the display is not receiving data to display and posts a Red X. Troubleshooting depends on which display has the red X. 

As discussed previously, DUs 1 and 2 are driven by IAC 1 DP 1 and DU’s 5 and 6 are driven by IAC 2 DP 2 (two displays driven by same Display Processor) so a red X on any single display in these locations cannot be an IAC (if the other display in the pair is good we can eliminate the IAC Display Processor). A HDLC/Wiring issue is most likely but not precluding a display with an internal connection failure (cold solder joint etc).

• If a red X is on DU 3 or DU 4 confirmation will be different because IAC 3 DP 1 normally drives DU 3 and IAC 3 DP 2 normally drives DU 4. 
• If DU 3 has a red X, turn SG 1 to ALTN on the Reversion Panel – this shuts down IAC 1 Display Processors and FWC 1 forcing IAC 3 DP 1 to pick up these displays, if they are good there is not a problem with the IAC but potentially HDLC/Wiring.
• If DU 4 has a red X, turn SG 2 to ALTN on the Reversion Panel – this shuts down IAC 2 Display Processors and FWC 2 forcing IAC 3 DP 2 to pick up these displays, if they are good there is not a problem with the IAC but potentially HDLC/Wiring.

A CHECK DU message when you see a good or valid display is a potential indication of a Wraparound Failure . The IAC Display Processor is generating the display data via HDLC but the FWC cannot confirm the data because Wraparound is missing.

 

Referencing the Schematic/Wiring Manuals, check Wraparound wiring. This is the only interface within the Electronic Display System that involves Junction Boxes. Wraparound from each DU is tapped at a Junction Box for routing to all FWCs.

Another potential cause is some data on a display might be misleading. 

Examples:

 

When a red X is posted on a single DU 1, 2, 5 or 6 it is logical that it cannot be a Display Processor as the DP drives 2 displays. IAC 1 DP 1 drives DUs 1 and 2. IAC 2 DP 2 drives DU’s 5 and 6. A possible cause could be HDLC / Wiring Issue or a bad connection in a display (failed display).

 

When a red X is posted on a single DU’s 3 or 4, it can be a Display Processor issue as IAC 3 DP 1 drives DU 3 and IAC 3 DP 2 drives DU 4. IAC 1 DP 1 drives DUs 1 and 2. IAC 2 DP 2 drives DUs 5 and 6.
It could be caused by a HDLC/ Wiring issue or a bad connection in a display (failed display).

If DU 3 has a red X, turn SG 1 to ALTN on the Reversion Panel – this shuts down IAC 1 Display Processors and FWC 1 forcing IAC 3 DP 1 to pick up these displays (DUs 1 and 2), if they are good there is not a problem with the IAC but potentially an HDLC/Wiring issue to DU3.
If DU 4 has a red X, turn SG 2 to ALTN on the Reversion Panel – this shuts down IAC 2 Display Processors and FWC 2 forcing IAC 3 DP 2 to pick up these displays, if they are good there is not a problem with the IAC but potentially HDLC/Wiring issue to DU 4.