VXI VT1422A - Manual (Page 100)

98 Programming the VT1422A for Data Acquisition and Control Chapter 4
Programmable Signal
Conditioning and EU
Conversion
The input and output SCPs are configured with the SCPI and/or VXIplug&play
programming. Analog SCPs are measured with the VT1422A’s A/D.
Configuring the analog SCPs include specifying what type of Engineering
Unit (EU) conversions are desired for each analog input channel. For
example, one channel may require a type T thermocouple conversion and
another may be a resistance measurement. The on-board Digital Signal
Processor (DSP) converts the voltage read across the analog input channel
and applies a high-speed conversion which results in temperature,
resistance, etc. Digital input SCPs perform their own conversions as
configured by the SCPI language.
Scan List and/or
C Language Control
Programming
The VT1422A can be used as either a conventional Scan List controlled
data acquisition unit with analog measurements automatically buffered and
available to the supervisory computer or the VT1422A can execute its own
internal ’C’ language algorithms which can perform data acquisition and
control and pass values to the supervisory computer when required.
Of course, both modes can be used for example when many analog data
acquisition channels need to be measured using the Scan List and one or
more algorithms are needed to perhaps monitor some of the data points
and make control decisions.
Runtime Remote
Scan Verification
The VT1422A provides a method to verify that remote channels in the scan
list defined in algorithms or with the ROUTe:SEQuence DEFine command
are successfully scanned in each RSCU. Special algorithm variables are
available to check the operating status of each VT1539A main channel. This
provides protection against an RSCU failing to scan remote channels due to
a signal cable disconnect or a power failure at the RSCU.
Operational
Overview
When the Trigger System is configured and either generates its own trigger
or accepts a trigger from an external source, an instrument operation cycle
begins. A simplified description of the cycle follows.
Acquire Input Values
All digital input SCP's latch their current input state and the A/D starts
scanning the analog channels specified in the Scan List with the
ROUTe:SEQuence:DEFine command or analog channels referenced by any
’C’ algorithms. All measurement data as seen by the ’C’ algorithms is
represented as 32-bit real numbers even if the input channel is inherently
integer (digital byte/word). The EU-converted numbers such as temperature,
strain, resistance, volts, state, frequency, etc. from Scan List channels
(ROUT:SEQ:DEF) are stored by default in the Current Value Table (CVT)
and the FIFO reading buffer. Values from analog channels referenced by ’C’
algorithms are stored in an Input Buffer and later accessed by the ’C’
algorithms executing on the VT1422A card.
Analog input values from channels in the Scan List, stored in the FIFO
and/or CVT can be read from the VT1422A without creating or running any
’C’ algorithms. This makes for easy traditional analog data acquisition
where no control aspect is required.