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Scanning Process and
Channel Timing
This section shows how to determine the channel update time and channel
autocalibration time. In addition, the scanning process is briefly described.
The RTD module channel update time is defined as the time required for the
module to sample and convert (scan) the input signal of an enabled input
channel and make the resulting data value available to the SLC processor for
update.
Channel Autocalibration
Upon entry into the channel enabled state, the corresponding channel is
calibrated and configured according to the channel configuration word
information. Channel calibration takes precedence over channel scanning
and is a function of the selected notch filter, as shown in the following table:
Table 4.C
Channel Calibration Time
Filter Frequency
Channel Calibration Time
10 Hz
7300 ms
50 Hz
1540 ms
60 Hz
1300 ms
250 Hz
388 ms
Update Time and Scanning Process
Figure 4.6 shows the scanning process for the RTD module assuming that the
module is running normally and more than one channel is enabled. The
scanning cycle is shown for the situation where channels 0 and 1 are enabled
and channels 2 and 3 are not used.
Important: The scanning process of Figure 4.6 is similar for any
number of enabled channels.
Channel scanning is sequential and always occurs starting with the lowest
numbered enabled channel and proceeding to the next highest numbered
channel, for example, channel 0 – channel 1 – channel 2 – channel 3 –
channel 0 – channel 1, etc. Channel scan time is a function of the filter
frequency, as shown in the following table:
Table 4.D
Channel Scan Time
Filter Frequency
Channel Scan Time
10 Hz
305 ms
50 Hz
60 Hz
250 Hz
The module–scan time is obtained by summing the channel–scan time for each enabled channel. For example, if
3 channels are enabled and the 50 Hz filter is selected, the module–scan time is 3
Chapter 4
Preliminary Operating Considerations
65 ms
55 ms
17 ms
65 ms = 195 ms.
4–9
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