Retiring the Card Cage: Distributed Real-Time PCIe I/O for Defense Automated Test Stands

A large defense industry contractor needed to modernize a fleet of Automated Test Stands built on legacy VXI/VME card cage architecture — and needed the replacement to carry over 30 PCIe cards across the fleet without giving up the deterministic timing that hard real-time test control demands. DuroPC's answer: a distributed fleet of rackmount systems linked into a single coherent real-time I/O domain over PCIe Non-Transparent Bridging.

The Challenge

The contractor's test stands relied on a wide range of I/O — avionics bus protocols (ARINC 429, MIL-STD-1553, ARINC 422), fiber and CAN interfaces, FPGA-based processing, and PXIe instrumentation — all coordinated under hard real-time control. VXI/VME architecture was purpose-built for smaller, message-based data transfers. As test requirements grew, the contractor needed an architecture suited to much larger DMA data throughput, without sacrificing the deterministic timing that IntervalZero RTX64 demands for real-time test control.

The scale requirement was significant: the fleet needed to support over 30 PCIe cards simultaneously, spanning multiple interface types — well beyond what a single chassis could accommodate.

The Approach

DuroPC worked with the contractor to replace the card cage architecture with a distributed, networked fleet of DuroPC rackmount systems, each densely populated with PCIe I/O cards. Rather than forcing all the cards into a single chassis, the design distributed I/O across multiple DuroPC units — while preserving a single, coherent real-time I/O domain across the fleet.

The key to making a distributed architecture behave like one deterministic system was the interconnect. DuroPC used a Dolphin PXH830 PCIe Non-Transparent Bridging (NTB) shared-memory device to link the chassis together. Unlike a conventional network fabric, PCIe NTB allows independent systems to share memory windows directly over a PCIe-based link — avoiding the latency and jitter that a standard Ethernet interconnect would introduce. This kept inter-chassis data transfer fast and predictable enough to meet IntervalZero RTX64's hard real-time requirements, even across a multi-chassis fleet.

Each DuroPC system was configured to run RTX64 with a minimum of 16 processing cores and as many PCIe lanes as the platform could support, ensuring headroom for the full I/O card complement:

• ARINC 429, MIL-STD-1553, and ARINC 422 interfaces
• Fiber and CAN interfaces
• x8 / x16 FPGA cards
• MXIe-to-PXIe chassis bridging
• Custom bus-interface card

Platform Evolution

The program has run across three generations of DuroPC hardware to date:

• First generation (discontinued): DuroPC RAC717 (5x PCIe x8, 11x PCIe x4 slots) and DuroPC R732 (17x PCIe x16, 1x PCIe x4 slots).
• Second generation (discontinued): DuroPC R5359-139988-XEON-1VD — a 4U rackmount platform with a 10x PCIe x16 / 4x PCIe x8 backplane, dual Xeon Silver 8-core/16-thread processors, 64GB DRAM, and dual removable SSD trays.
• Current generation: DuroPC RX817-4 host system paired with the DuroPC EXP500-16 expansion chassis.

The RX817-4 is a 4U rackmount platform built on 4th/5th Gen Intel Xeon Scalable processors with a 3x PCIe x16 / 2x PCIe x8 / 1x PCIe x4 / 1x PCIe x1 backplane (all Gen 5 signaling on the x16/x8 slots). Each RX817-4 can be paired with one or more EXP500-16 units — a 16-slot PCIe x16 Gen 4 expansion chassis connected via a Gen 5 host interface card over a 32GB/s uplink — multiplying available PCIe slot density per host well beyond what the earlier platform generations could support in a single chassis.

Each transition in the program has been driven by end-of-life on the prior generation rather than a design limitation. The move to RX817-4 and EXP500-16 also improves the host-to-expansion slot ratio, reducing the number of DuroPC chassis needed to reach the same distributed PCIe card count across the fleet, while remaining fully compatible with the NTB-linked, RTX64-scheduled architecture established in earlier generations.

The Result

The contractor now runs Automated Test Stands built on a distributed, PCIe-based architecture supporting all the PCIe cards across the fleet at any given time — replacing legacy VXI/VME infrastructure with a platform suited to modern DMA-heavy test workloads, while preserving the deterministic real-time performance IntervalZero RTX64 requires.

The program's current DuroPC RX817-4 and EXP500-16 platform carries this architecture forward with higher per-chassis PCIe slot density, supporting continued fleet growth without disrupting the underlying NTB-linked, RTX64-scheduled design.

Customer identity withheld per defense-sector disclosure policy.