Technical Notes

Data Acquisition Handbook

Introduction

The purpose of this guide is to provide the reader with an overview of important considerations in the design and implementation of high performance data acquisition systems and how KineticSystems products can be configured to meet some of the more demanding applications. The product designers at KSC have gone to considerable effort to desig n a family of VXI modules which provide a wide range of extensibility in the design of high performance systems. It is important to understand how the features of the various system components interact and can be brought together to provide unique system capabilities.

This document goes beyond a functional description of the individual modules and illustrates some of the unique features that have been designed into the system components. KineticSystems manufactures a wide variety of modules that are appropriate over a wide range of applications. This guide focuses on a subset of modules that play a critical role in demanding high performance applications. Not all applications require high performance, and many high performance applications have components that are low or medium performance that can be more cost effectively solved with other components. For this reason the data acquisition systems designer should be acquainted with the full product line and not just the limited products discussed here.

Finally there is the question of just what is "high performance?" There are many answers to this question and one needs to keep in mind that while one thermocouple channel with a 10 Hz sample rate may not be "high performance," 1000 channels at 10 Hz with 3 microvolt accuracy may be something else.

VXIbus Overview

The choice of a modular backplane standard for I/O can be an important consideration. VXI is an industry standard with multiple manufacturers. It is an exte nsion of the VME bus standard for Instrumentation. For purposes of this document we will focus on the C-size VXI. Some of the features it brings to the user over VME are as follows:

• The VXI standard provides a number of chassis-wide timing features that can be particularly valuable in synchronizing data acquisition activity among multiple modules. These features can be particularly valuable in applications where it is important to synchronize data acquisition activity across multiple mo dules. For C-size VXI these include:

  - Common precision 10 MHz clock that can be used as a common clock to synchronize activity within modules.

  - Eight TTL and two ECL "trigger lines" for inter-module synchronization such as sample clocks and event triggering.

• The VXI standard provides a 12-bit "local" bus between adjacent modules that can be used to pass analog or digital information between cooperating modules. This bus frequently eliminates the need for "messy" wiring b etween modules making for a cleaner looking configuration with significantly improved system reliability.
  
  
• The VXI standard provides a geographic addressing capability. This can be important since wiring is more easily associated with a physical location (slot) than by module type and it's associated options and address strapping-especially when the address strapping is not visible from the front panel. Be sure your VXI vendor's modules support this feature.
  
  
• The shielded module pr ovides good EMI shielding for sensitive analog circuitry which is necessary for good accuracy.
  
  
• The deeper card allows the designer to place analog circuitry further from the high-speed digital backplane, further reducing noise sensitivity.
  
  
• Chassis cooling specifications of the VXI standard insures a good uniform cooling environment which minimizes thermal drift and gradients. High-gain analog circuitry is generally sensitive to the thermal environment which affects stability and channel-to-channel measurement accuracy particularly with thermocouples and other low-level analog signals.
  
  
• The VXI standard provides Module ID registers that allow software to verify that it is addressing the proper module. Modules are identified by a unique manufacturers ID, model number and hardware/firmware revision level. This can avoid costly mistakes resulting from taking data from an inappropriate module. It also enables software to verify system configuration-especially when combined with geographic addressing.
  
  
• The VXI standard provides for each module, as appropriate, to perform any self-diagnostics during power-up and provide pass/fail status bits. This feature reduces the chances of attempting to take data from a failed module. Some features unique to the KineticSystems second generation VXI products include:
  
  
• Automated channel-by-channel calibration and system checkout is an important tool in data acquisition. KSC modules implement the capability to inject either an internal or external precision reference into each channel for calibration and end-to-end system checkout.
  
  
• Many VXI modules have multiple options available. KSC modules implement a module option ID in addition to the standard manufacturers ID and model number for more complete module identification.
  
  
• In many cases users have a need to track certain critical information regarding particular modules. KSC modules implement a user-writeable EEPROM for this purpose.
  
  
• Signal conditioning is an important consideration in many data acquisition applications. To provide maximum flexibility in signal conditioning KSC has developed a powerful new architecture that makes extensive use of the VXI Local Bus to interconnect the ADC to a family of signal conditioning modules on a mix and match basis. The architecture provides a clean analog environment at the signal conditioner with minimum digital circuitry to introduce noise in low-level front-end circuits and sufficient bo ard real estate to provide a wide range of signal conditioning capabilities with no inter-module wiring mess.
  
  
• KSC modules make full use of VXI trigger lines for inter-module synchronization and clocking. The KSC standard is to provide full flexibility to the user in trigger line assignment and usage and also provides external clocking and triggering capability.

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