VxWorks 6.x CCU/ERM Fixes: DOS Boot, UDP, FTP, DBR and LBA
Rail transit CCU and ERM systems running VxWorks 6.x often exhibit startup instability and version-specific I/O anomalies. A common field failure presents as a six-LED lockup during power-on, indicating that the system fails before reaching the VxWorks shell.
This article consolidates root-cause analysis and validated fixes for DOS-induced FAT32 corruption, UDP boot-time failures, FTP persistence issues, and DBR/LBA constraints in VxWorks 6.9.
🔍 Platform and Failure Characteristics #
The CCU/ERM hardware platform is based on an AMD LX800 PC/104-Plus board with IDE/EDOM storage and standard industrial I/O. Software typically includes VxWorks 6.x with application layers such as OpenPCS.
Observed Failure Mode #
- All six front-panel LEDs remain permanently lit
- No VxWorks shell or network services available
- Failure reproducible under repeated power cycling
The issue is consistently linked to early boot-stage failures.
🛠️ Eliminating DOS-Induced FAT32 Corruption #
Repeated power cycling reveals corruption in FAT32 directory entries, specifically affecting critical boot files such as io.sys and bootrom.bin.
Root Cause #
DOS modifies directory entries during boot (e.g., updating bootlog.txt). If power is interrupted during these writes:
- Directory metadata becomes inconsistent
- File cluster pointers are corrupted
- Bootloader fails to locate VxWorks image
Solution: Direct FAT32 Boot via Custom DBR #
Replace the standard DOS boot path with a custom DBR that directly loads bootrom.sys.
Key Implementation Points #
- Custom DBR includes boot code and valid BPB
- File lookup logic directly locates
bootrom.sys - DOS layer is completely bypassed
This approach removes dependency on DOS file operations and eliminates corruption risk. Field validation shows stable cold-start behavior with no recurrence of LED lockups.
📡 UDP Unicast Boot-Time Failure #
A network failure scenario occurs when the Ethernet cable is connected after system boot.
Symptoms #
- Network services fail if cable is inserted after a delay
sendto()returns error under active UDP task conditions- Other services recover when UDP task is disabled
Root Cause #
This behavior is specific to VxWorks 6.5 and relates to network stack initialization under delayed link conditions.
Resolution #
- Upgrade to VxWorks 6.8 or 6.9
- No reliable patch exists for 6.5
Later versions resolve the issue at the stack level.
📤 FTP File Persistence and Directory Performance #
Persistence Issue #
Files transferred via FTP or standard I/O do not persist after reboot.
Root Cause #
The dosFs cache flush mechanism is disabled by default, preventing buffered writes from committing to disk.
Fix #
Enable background cache flushing:
Dos FS Cache Handler -> enable background flush task for dosFs cache = TRUE
This ensures consistent data persistence across restarts.
Directory Performance Issue (VxWorks 6.8) #
- Large directories (>200 files) cause slow refresh
- Rapid operations may trigger kernel exceptions
Resolution #
- Upgrade to VxWorks 6.9
- Improved directory handling removes instability
🔧 DBR Validation and LBA Access in VxWorks 6.9 #
DBR Recognition Issue #
After deploying a custom DBR, VxWorks 6.9 may fail to detect the filesystem.
Root Cause #
Strict validation of the OEM name field in the BPB.
Fix #
- Set a valid OEM identifier in DBR header
- Ensure compatibility with VxWorks parser expectations
LBA Sector Access Limitation #
Low-level disk access via ataRawio() is limited to early sectors.
Observed Behavior #
- Sectors beyond a threshold cannot be accessed
- Same limitation exists across multiple VxWorks versions
Practical Impact #
- Direct sector patching is not viable for field updates
Recommended Approach #
- Perform full disk image updates
- Apply DBR fixes during image preparation
📌 Conclusion #
VxWorks 6.x CCU/ERM issues are primarily caused by fragile boot dependencies and version-specific driver limitations. Removing DOS from the boot chain, enabling dosFs cache flushing, and upgrading to newer VxWorks versions resolve the majority of failures.
The most critical improvement is the adoption of a direct FAT32 boot mechanism, which eliminates directory corruption and restores deterministic startup behavior. Combined with proper system configuration and version alignment, these fixes provide a reliable and scalable foundation for long-term deployment in rail transit systems.