VXI SVM2608 user manual Fifo Mode, Calibrations, Test Bus, Commands

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FIFO Mode

In FIFO mode, the user can retrieve data from the board as acquisition progresses. The memory behaves as a FIFO: data is written into a circular buffer with new data overwriting older data when the buffer is full. A Threshold Flag is available to monitor the status of the buffer and prevent overwriting the data or under-reading it.

The Sample Points register that is used in Linear mode to determine the amount of data to be captured is used in FIFO mode to determine the size associated with the FIFO Threshold Flag. When the number of samples stored in memory equals the number of points set in the Sample Points register, the FIFO Threshold flag is asserted. In this manner, the user can wait until a certain number of samples are captured before they download data from the board. If the user fails to retrieve the data from the card in time and new data overwrites older data, then the FIFO Overrun flag is asserted. Conversely, if the user attempts to read more data than has been stored, the FIFO Underrun flag is asserted. The FIFO Threshold flag is cleared when data is read from the board and the total amount of “new (unread)” data in the buffer is less than the THRESHOLD value. The FIFO Overrun and Underrun flags are cleared only when a new acquisition is initiated.

Calibrations

Due to the nature of the semiconductors and passive components, not all parts have exactly the same characteristics. Slight differences exist from component to component. While these inconsistencies are unavoidable, they do not affect the basic functionality of the electronic instrumentation. The precision of the instrument, however, can be altered by these variances.

One way to eliminate these slight variations is to use expensive, precision parts or to perform a rigorous parts selection procedure to ensure consistency. These measures, however, would dramatically increase the cost of the board. Another way to compensate for offset and gain variations is to take a number of measurements using precision calibrated instruments of known voltage and resistance. Their known values are then compared against the values attained for each channel and the difference is used to adjust future measurements. These adjustments are called calibrations. They are performed at the factory using approved calibration sources.

Test Bus

The SVM2608 is capable of performing a self-test to check for functionality and accuracy. Using a local voltage reference source and local resistance references, basic function tests can be performed. Four different voltage reference sources are available on the board: ±9.45 V and ±0.945 V. Two Resistance References are available: 128 Ω and 81.92 kΩ. Two different signal generators can also be used for different tests: a RAMP generator and a PULSE generator. Any of these locally generated test sources can be placed on the internal Test Bus (TB). The Test Bus can then be connected to the input of any or all of the channels. Only one of the test signals can be connected to the Test Bus at one time. The test sources can be connected to the Test Bus using microprocessor commands. The Test Bus is also available to the user for monitoring on pins 24 and 13 of the Front Panel Connector. (See Front Panel Interface Wiring for more detail.)

The self-test is performed by sending a command to the microprocessor, instructing it to run the

self-test (see Microprocessor3Commands). When the microprocessor runs the self-test, a Test Result is returned (see the description of the Self Test Command for a more detailed description).

Commands

The SVM2608 is equipped with a processor. While the processor is not directly involved in the acquisition process, its presence on the board significantly enhances the capabilities of the SVM2608 digitizer.

The user can choose to download the data on to a CPU and perform custom data processing, or they can instruct the on-board microprocessor to perform one or several predefined calculations

SVM2608 Introduction 15