Spectrum Controls 1769 Series User Manual page 40

3-10 Chapter 3: Module, Data, Status, and Channel Configuration
User's Manual Pub. 0300244-02 Rev. A
The update time can slow down for channels 0 and 5 during CJC sampling which
occurs once every 10 seconds. At the 10 second internal the ADC switches from
measuring the thermocouple voltage to measuring the CJC voltage. Once a
sample of the CJC voltage is taken, the ADC will switch back to measuring the
thermocouple voltage. This switching process increases the channel update time.
Table 3-5 shows the increased update time for channels 0 and 5 during the CJC
sampling process. After the CJC sampling process completes, channels 0 and 5
return to the, faster, normal conversion mode as described above.
For channels 1, 2, 3 and 4 the update rate is fixed and unaffected by the CJC
measurement. Likewise, if all channels are configuring for measuring millivolts,
then the CJCs are never sampled and the module update time is equal to the
slowest configured channel update time.
Channel update time is also dependent upon the input filter selection. Table 3-5
shows the channel update times based on filter selection.
Example 1 Channels enabled for different input types
Channel 0 Input: ±50 mV with 4.17 Hz Filter
Channel (1 – 5) Input: Type J TC with 10 Hz Filter
Module update time before CJC update
= slowest channel update time
= 246 ms
Module update time during CJC update
Channel 5 update time changes to 500 ms
= slowest channel update time
= 500 ms
Example 2 All Channel enabled for thermocouple inputs
Channel (0 – 5) Input: Type J TC with 470 Hz Filter
Module update time before CJC update
= slowest channel update time
= 8 ms
Module update time during CJC update
Channel 0 and 5 update time changes to 24ms
= slowest channel update time
= 24 ms
Example 3 All Channel enabled for millivolt inputs
Channel (0 – 5) Input: ±50 mV with 470 Hz Filter
CJCs not scanned
= slowest channel update time
= 8 ms