T&M VME Bus vs. PXI Bus: Key Differences Explained
This article delves into the comparison between VME (Versa Module Europa) bus and PXI (PCI eXtensions for Instrumentation) bus, outlining their distinct characteristics and applications.
VME Bus: A Robust Multiprocessing Platform
The VME bus was designed as a robust platform specifically to support multiprocessing, a feature crucial for controllers akin to computer boards. Its hardware handles arbitration effectively, eliminating software overhead. Notably, the VME bus operates asynchronously. Here’s a rundown of its core features:
- Stable platform: Known for its reliability and consistent performance.
- Good interconnect (backplane): Offers solid connections between modules.
- Universal CPU board usage: The same CPU board can be used across various slots.
- Good interrupt structure: Manages interrupts efficiently.
- Effective multiprocessing: Supports multiprocessing without relying heavily on software.
- 21 slots per chassis: Provides a generous number of expansion slots.
- Data throughput: Achieves speeds over 700 Mbps.
- No standardized timing and synchronization: Lacks precise timing and synchronization features.
- Real-time support: Can handle real-time applications.
PXI Bus: The Modern Choice for Instrumentation
PXI, short for PCI eXtensions for Instrumentation, is a rugged, PC-based platform geared towards test and measurement, and automation systems. It cleverly combines the electrical characteristics of the PCI bus with the robust, modular Eurocard packaging of compact-PCI. It also includes specialized synchronization buses and software. The PXI bus operates synchronously, offering high performance at a lower cost. The PXI Systems Alliance (PXISA), with over 70 member companies, governs the PXI standard. Here’s what makes PXI stand out:
- Open multi-vendor standard: Supported by over 70 vendors with more than 1200 modules available.
- Plug and Play: Offers easy setup and configuration.
- Extensive I/O and instrumentation choices: Provides a wide array of I/O options and instrumentation modules.
- Fast bus transfer and low latency: Delivers swift data transfer with minimal delay.
- Off-the-shelf bus software drivers: Ready-to-use software drivers for easy integration.
- 21 slots per chassis: Similar to VME, offering 21 slots per chassis.
- Data throughput: Exceeds 6 Gbps.
- Precise timing and synchronization: Offers timing and synchronization with picosecond-level accuracy.
- Real-time support: Supports real-time operations with dual-core processors and beyond.
- Continuous processor advancements: Adapts to new processor technologies like dual-core and quad-core.
- Versatile bus connectivity: Can connect to various buses like GPIB, USB, and LAN.
The PXI platform includes two main bus types: PCI and PCI Express, offering further flexibility and performance.
VME Bus vs PXI Bus: A Tabular Comparison
To highlight the key differences, here’s a table summarizing the comparison:
| Features | VME | PXI |
|---|---|---|
| Data Throughput (Max.) | 700 Mbps | System: 6 Gbps, Peripheral Slot: 2 Gbps |
| EMI/EMC, Electrical Safety | No | Yes (section 3.9.1 of spec) for EMC testing, Yes (section 3.9.2 of spec) for electrical safety |
| Cooling | Defined Air Flow: No | Defined Air Flow: Yes |
| Timing and Synchronization | Synchronization Clock Trigger Lines: No | Synchronization Clock Trigger Lines: Yes |
| Power Voltages | +5, -5.2, -2, +/12, +/-24 | +5, +3.3, +/-12 |
| Communications | Standardized protocols: Yes, message-based, Register-based | Yes, register-based, message-based, peer-to-peer bus mastering peripherals |
Conclusion
In summary, while the VME bus offers a stable platform for multiprocessing with a good interrupt structure, the PXI bus provides a modern, higher-performance alternative with significantly faster data transfer rates and precise timing and synchronization capabilities. The choice between VME and PXI depends on the specific requirements of the application, with PXI often being favored for its versatility and performance in test and measurement systems.
