Theory of Operation
Chapter 4
SCXI-1124 User Manual
4-2
© National Instruments Corporation
The major components of the SCXI-1124 are as follows:
•
The rear signal connector
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The SCXIbus connector
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The SCXIbus interface
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The digital control circuitry
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The analog output channels
The SCXI-1124 has six isolated output channels. Each channel can be configured for one of six
voltage ranges or 0 to 20 mA output. The module is programmed via the SCXIbus and provides
interface circuitry to allow a DAQ board to control the SCXIbus. An EEPROM provides
calibration information for all the DAC channels. The rest of this chapter explains the theory of
operation of each of the SCXI-1124 components.
Rear Signal Connector, SCXIbus Connector, and SCXIbus Interface
The SCXIbus controls the SCXI-1124. The SCXIbus interface interfaces the signals of the rear
signal connector to the SCXIbus, allowing a DAQ board to control the SCXI-1124 and the rest of
the chassis.
Digital Control Circuitry
The digital control circuitry contains a Module ID register, write circuitry for updating the
DACs, and an EEPROM for storing calibration constants.
The Module ID register contains the module ID 14 hex, a code unique to the SCXI-1124. You
can read this module ID over the SCXIbus to determine the type of module in a particular slot.
The write circuitry writes the range and DAC bits to the desired DAC channel. You can only
write to one channel at a time. Multiple channels cannot be updated simultaneously.
The EEPROM stores calibration constants for the six voltage ranges and the 0 to 20 mA range
for all six channels. Information in the EEPROM is retained when the module is turned off. The
SCXI-1124 has calibration constants already stored in the EEPROM. You can modify these
constants for your own set of operating conditions. One set of constants is reserved and cannot
be modified except at the factory, which ensures that you will not accidentally erase the default
calibration constants.
These constants are used in a linear interpolation algorithm to determine the bit pattern to write
to a DAC channel when a particular voltage or current is desired. For more information on the
EEPROM and calibration, see Chapter 5, Calibration.
