Application Programs 95
Application 7. Output Sequencing Without Computer Intervention
Overview Of Application
When characterizing devices, the DUT’s performance is measured over a range of power supply voltages. This test can be
performed without computer intervention by using hardware signals from the measurement instrument to cause the power
module to sequence to the next voltage in a preprogrammed List. By buffering these readings in the measurement
instrument, the entire test can be executed without computer involvement. For characterizations that require long
measurement times, the computer is free to do other tasks. For characterizations that must execute at hardware speeds, the
computer is not involved and will not slow down the test loop.
In this example, the power module must apply 8 to 14 volts (in 13 0.5-volt increments) to an automotive engine sensor. The
module varies the bias voltage to the engine sensor and the sensor’s output is measured to characterize its performance over
the range of possible "battery voltages". The sensor output is measured by a DMM that has an internal buffer and stores
each reading.
By combining Lists and trigger capabilities, the MPS can be used to address this application. The module can be
programmed to use its triggering capabilities to the fullest extent. Each time the module executes the next step in its List
and changes voltage, the module will generate an external trigger. The external trigger will cause the DMM, equipped with
an external trigger input, to take and store a reading. The DMM, also equipped with a "Measurement Complete" output,
sends its output trigger signal to the module to tell the module to go to its next List point. Effectively, the module and the
DMM "handshake", so that the two function at hardware speeds without computer intervention.
When the test is complete, either device can signal the computer to get the data from the DMM. For the purpose of this
example, the module will generate an SRQ when the last List point has been executed. This is indicated by the OPC
(Operation Complete) bit in the status register.
Another detail that needs attention is timing. The DUT may require some settling time before the DMM is told to take a
reading. The module’s dwell time can be used to do this. The STC (Step Complete) status signal indicates when the point
has been executed and its dwell time has expired. The dwell time is programmed to be the engine sensor’s settling time.
The external trigger is generated when STC is asserted. Thus, the DMM will not be triggered until the dwell time has
expired and the sensor’s output has settled.
This type of self-paced test execution is useful in two situations. When the test must execute very fast, there is no time for
the computer to be involved in each iteration of the test loop. Therefore, the test must execute without computer
intervention. The second situation is when the test is very long. For example, if the measurement instrument took 1 minute
to make each measurement, the test would take 13 minutes to execute. The computer is not used efficiently if it is idle while
waiting for each measurement loop, so it would be best to have the computer executing another task. Without self-pacing,
you would need to develop interrupt driven software that stops every 1 minute to take a reading. By letting the module and
the DMM run on their own, code development is much simpler and computer resources are used more efficiently.
MPS Features Used
• 20-point voltage List.
• Dwell time.
• Trigger-paced Lists.
• Generate an SRQ on a change in internal status.
• Generate a trigger on a change in internal status.
• Trigger in/out from MPS mainframe backplane TTL Trigger.
• Trigger on a GPIB trigger command.