Measurement Computing CIO-DAS-TC User Manual
Page 14
CIO-DAS-TC User's Guide
Functional Details
14
Processing and control
This section consists of control and decode logic, a microcontroller and local memory to perform channel
scanning, CJC measurements, calibration, linearization, averaging, and voltage/temperature translation.
The above parameters are set up from a configuration file which is downloaded by the PC to the
microcontroller’s local memory through the Dual Port RAM. After the microcontroller is given the
command to start conversions, these parameters are set on a channel-by-channel basis with data reported
to the PC in the format specified by the configuration file. For thermocouple inputs, the microcontroller
reads the counter, adjusts the data based on the CJC value and gain/offset calibration, then linearizes and
converts the reading to the appropriate temperature units.
To perform linearization, the microcontroller gets the raw frequency count from TIMER0, translates that
into bits, factors in the CJC correction and gain/offset calibration, then refers to a previously stored
lookup table stored in ROM. There is a separate table for each thermocouple. The lookup tables are a
method to optimize the linearization by using more reference points along areas of greatest
temperature/voltage change instead of using mathematical translation, which requires lengthy polynomial
manipulation. Using lookup tables requires finding two consecutive points, one greater and one less than
the measured value, then interpolating the measured temperature value.
Process flow
The PC itself performs very few functions for the CIO-DAS-TC. The driver software included with the
CIO-DAS-TC sets up individual channels, including the thermocouple type, CJC on/off, voltage or
thermocouple gain, channel, and temperature units. The sample rate and sample averaging configuration
are also set by the driver for all channels. Both during initialization and when the configuration changes,
this information is passed to the CPU through the Dual Port RAM and stored for the specified channel.
The PC then notifies the CPU to start taking measurements. When the CPU completes a conversion, an
interrupt is generated so that the PC reads the data from the Dual Port RAM which the CPU had written
to. The 32-bit floating point data is stored in four consecutive locations in the Dual Port RAM. Refer to
the "Dual Port RAM Memory Map" section in the Register Map for the CIO-DAS-TC for more details on
this process. This document is available on our web sit
DAS-TC.pdf
.
The onboard CPU sets all of the parameters for conversion of the selected channel. After conversion, the
CPU retrieves the data, adjusts the data based on the stored CJC measurement, calibrates the data against
gain/offset error, linearizes the data based on lookup tables for each associated thermocouple type, and
reports the data to the PC through the Dual Port RAM. During this process, the CPU goes to the next
channel and sets up the parameters for that channel to allow sufficient settling time before the next
conversion begins.