Rockwell Automation 1746-NR8 SLC 500 RTD/Resistance Input Module User Manual - Manual (Page 46)

Publication 1746-UM003A-EN-P
3-10 Preliminary Operating Considerations
Channel Autocalibration
Upon entry into the channel enabled state, the module configures that channel
and performs an autocalibration on the module if the combination of input
and excitation current are unique to that channel. Module calibration takes
precedence over channel scanning. Module calibration time is dependent on
the number of unique input type and excitation current combinations and is
equal to 510 msec +(125 msec x number of unique combinations).
Update Time and Scanning Process
The illustration on page 3-11 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 through 7 are not used.
Channel scan time is a function of the filter frequency, as shown in the
following table:
Table 3.3 Channel Scan Time
The fastest module update time occurs when only one channel with a 6400 Hz
filter frequency is enabled and lead resistance measurement is disabled.
Module Update Time = 6 ms
With 3 channels enabled, the module update time is:
3 channels x 6 ms/channel = 18 ms. The slowest module update time occurs
when eight channels, each using a 28 Hz filter frequency and with lead
resistance measurement always enabled.
Module Update Time = 8 x 250 ms = 2000 ms
Filter Frequency
Channel Scan Time
(1) With Lead Resistance
28 Hz 125 ms 250 ms
50/60 Hz 75 ms 147 ms
800 Hz 10 ms 18 ms
6400 Hz 6 ms 10 ms
(1) The module-scan time is obtained by summing up the channel-scan time for each enabled channel. For
example, if 3 channels are enabled with lead resistance and the 50/60 Hz filter is selected, the
module-scan time is 3 x 147 ms = 441 ms.