VXI vs. AXIe: Modular Test System Architecture
Filed under Modules, misc.
Transitioning from legacy test architectures to modern standards is a critical consideration for test system designers and engineers. The choice between VXI (VME eXtensions for Instrumentation) and AXIe (AdvancedTCA Extensions for Instrumentation and Test) represents a major technological shift. While VXI was long the standard for large-scale, instrument-on-a-card systems, modern test demands have pushed systems toward AXIe's high-speed serial backplanes, improved power distribution, and advanced triggering. This guide examines the mechanical, bandwidth, and synchronization differences between these two architectures, using representative hardware like the Keysight Technologies (Agilent HP) E1406A VXI command module and the Keysight Technologies (Agilent HP) M9505A AXIe chassis to illustrate the differences.
- AXIe Host Bandwidth
- Up to 4 GB/s via PCIe Gen 2 x8 link
- AXIe Slot Power Capacity
- Up to 200 W per slot
- Legacy VXI Interfaces
- GPIB (IEEE-488), RS-232, VXIbus
- AXIe Trigger Lines
- 12 AXIe trigger bus lines
Backplane Bandwidth and Communication Topology
The primary distinction between VXI and AXIe lies in how data moves across the backplane. Legacy VXI systems are built on parallel VMEbus architecture. Communication with a host PC typically requires a Slot 0 command module, such as the Keysight Technologies (Agilent HP) E1406A. The Keysight Technologies (Agilent HP) E1406A utilizes a 16 MHz 68070 processor, up to 2 MB of RAM, and translates high-level SCPI commands to register-based instruments. It connects to the host via GPIB or RS-232, which imposes limits on data throughput.
In contrast, AXIe platforms utilize high-speed PCI Express (PCIe) serial backplanes. In an AXIe 1.0 system like the Keysight Technologies (Agilent HP) M9505A, the backplane features a PCIe Gen 2 link to the host (x8 configuration), providing up to 4 GB/s of throughput via a cable connector. To individual slots, it provides a PCIe Gen 2 x4 link supporting up to 2 GB/s per slot. This architecture dramatically reduces data transfer bottlenecks during intensive testing cycles.
Triggering and System Synchronization
Timing and synchronization differ significantly across the two standards. VXI hardware utilizes a 10 MHz reference clock. With a command module such as the Keysight Technologies (Agilent HP) E1406A, engineers can synchronize this VXI backplane 10 MHz clock to an external TTL signal to coordinate measurements across cards.
AXIe advances synchronization capabilities by integrating dedicated timing resources directly into the backplane architecture. An AXIe-1.0 chassis like the Keysight Technologies (Agilent HP) M9505A provides 12 dedicated AXIe trigger bus lines. This allows for precise, multi-channel instrument coordination across the 5-slot chassis without requiring external wiring, which simplifies the development of complex, multi-instrument test routines.
Mechanical Footprint and Power Management
Physically, VXI and AXIe modules have distinct footprints and power requirements. VXI instruments are typically designed as C-size modules, where a controller like the Keysight Technologies (Agilent HP) E1406A occupies a single slot to manage resources.
AXIe systems optimize space by integrating essential system management hardware. For example, the 4U-high Keysight Technologies (Agilent HP) M9505A features an integrated system module that does not occupy any of the 5 instrument slots, leaving all 5 slots available for measurement modules.
Power delivery is also much higher in AXIe configurations. While legacy VXI slots have strict power limits, AXIe slots in the Keysight Technologies (Agilent HP) M9505A can deliver up to 200 W of power per slot, accommodating high-performance instrumentation cards that demand superior cooling and power capabilities.
Example instruments
Frequently asked questions
- What function does a Command Module serve in legacy VXI systems?
- In VXI systems, command modules like the Keysight Technologies (Agilent HP) E1406A function as a VMEbus system controller and a Slot 0 / Resource Manager. They configure the VXI backplane, manage system communication, and translate standard SCPI commands to coordinate register-based instruments.
- How does AXIe handle system management without taking up instrument slots?
- Some AXIe chassis, such as the Keysight Technologies (Agilent HP) M9505A, include an integrated system module built directly into the frame. This architecture manages backplane communication and host connectivity (such as LAN, USB 2.0, and PCIe Gen 2 x8) without occupying any of the dedicated instrument slots.