Output Latency of PXIe-6569

Updated Mar 12, 2026

Issue Details

Users of the PXIe‑6569 may encounter non‑deterministic or variable data latency when measuring the timing relationship between an input clock edge and the corresponding data output. Observed latency values may vary from acquisition to acquisition (for example, measurements differing by ~1.6 ns or ~3.2 ns), which can appear as inconsistent FIFO or clock‑to‑data behavior.
 
This behavior is most commonly reported when the PXIe‑6569 is used in custom CLIP implementations or in applications that require fixed, cycle‑accurate, or bit‑to‑bit deterministic latency. In such use cases, customers may expect a constant alignment between specific data bits (for example, bit 0 to bit 0) across clock cycles, which the PXIe‑6569 does not guarantee.

Solution

The observed latency variation on the PXIe‑6569 is expected behavior and inherent to the module’s high‑bandwidth LVDS design. To maintain signal integrity at high data rates, the PXIe‑6569 implements bit slip in its LVDS/SerDes data path, allowing incoming data to be captured on different bit positions within a byte. While the core FPGA datapath latency is consistent (approximately 64 ns clock‑to‑clock), the specific bit within the byte that is latched may vary from one acquisition to the next. This bit‑level alignment variability results in apparent latency shifts that fall within the width of a byte and do not indicate changes in the underlying logic delay.
 
 
Because the PXIe‑6569 relies on a SerDes CLIP that is not designed to provide deterministic bit‑to‑bit or frame‑to‑frame latency, fixed clock‑to‑data alignment cannot be guaranteed. As such, the PXIe‑6569 is not recommended for applications that require deterministic or cycle‑accurate latency, and is best suited for use cases where high throughput and signal integrity are the primary requirements rather than precise latency determinism.

Related Links

PXIe-6569 Getting Started Guide - NI

How to Adjust the DI Read Timing and the Clock Edge Relationship for PXIe-6569? - NI

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