Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 64)

Publication 1762-UM002A-EN-P - July 2002
3-34 Module Data, Status, and Channel Configuration
Channel update time is dependent upon the input filter selection. The
following table shows the channel update times.
The CJC input is only sampled if one or more channels are enabled
for any thermocouple type. The CJC update time is equal to the largest
channel update time of any of the enabled thermocouple inputs types.
In that case, a single CJC update is done per scan. See the scan
diagram on the previous page. The cyclic calibration time only applies
when cyclic calibration is enabled and active. If enabled, the cyclic
calibration is staggered over several scan cycles once every five
minutes to limit the overall impact to module update time.
Effects of Autocalibration on Module Update Time
The module’s autocalibration feature allows it to correct for accuracy
errors caused by temperature drift over the module operating
temperature range (0 to 55°C). Autocalibration occurs automatically
on a system mode change from Program-to-Run for all configured
channels or if any online
(1)
configuration change is made to a
channel. In addition, you can configure the module to perform
autocalibration every 5 minutes during normal operation, or you can
disable this feature using the Enable/Disable Cyclic Calibration
function (default is enabled). This feature allows you to implement a
calibration cycle anytime, at your command, by enabling and then
disabling this bit.
If you enable the cyclic autocalibration function, the module update
time increases when the autocalibration occurs. To limit its impact on
the module update time, the autocalibration function is divided over
multiple module scans. The first enabled channel receives an A/D
converter (ADC) self-calibration and a channel offset calibration over
the course of two module scans. The time added to the module
update time depends on the filter selected for the channel, as shown
in Table 3.6 on page 3-35. Each additional enabled channel receives
Table 3.5 Channel Update Time
Filter Frequency Channel Update Time
10 Hz 303 ms
50 Hz 63 ms
60 Hz 53 ms
250 Hz 15 ms
500 Hz 9 ms
1 kHz 7 ms
(1) During an online configuration change, input data for the affected channel is not updated by the module.