Unsynchronized Readings From Multiple PXIe-4339 Modules in FlexLogger Compared to LabVIEW

Issue Details

When using two PXIe-4339 modules in the same chassis with FlexLogger 2024 Q4, sourcing the same analog output signal to both AI channels results in unsynchronized readings. However, performing the same operation in LabVIEW using the NI-DAQmx API yields synchronized results. 

Solution

This issue is caused by channel-expanded tasks and how FlexLogger internally rewinds the read buffer after reading the first sample. NI has filed a bug for this issue and plans to release a fix future release of FlexLogger.

Workaround: Prevent Channel Expansion

Channel expansion occurs when:

• Multiple modules of the same type are in the same chassis
• All channels are configured with the same sample rate

To avoid this, configure each PXIe-4339 module with slightly different sample rates:

1. Slow: 255 Hz
2. Medium: 256 Hz
3. Fast: 257 Hz

This allows up to three PXIe-4339 modules in the same chassis to operate without channel expansion. Even though the sample rates differ, synchronization can still be achieved using a start trigger and shared reference clock.

Note: Channel expansion does not occur across chassis. Identical sample rates can be used for modules in different chassis without issue.

Additional Information

This workaround is not ideal for large systems in FlexLogger, as it limits the number of identical modules per chassis to three.