Rockwell Automation 1762-IR4 RTD/Resistance Input Module - Manual (Page 65)

Publication 1762-UM003A-EN-P - February 2003
Module Data, Status, and Channel Configuration 3-31
Calculating Module Update Time with Cyclic Lead Wire
Compensation Enabled
The following example illustrates how to determine module update
time with cyclic lead wire compensation enabled.
EXAMPLE
Two Channels Configured with Cyclic Lead Wire Compensation Enabled
Channel 0 Input: 100
Ω
Platinum 385 with 60 Hz Filter (use 60 Hz filter for lead wire)
Channel 1 Input: 100
Ω
Platinum 385 with 250 Hz Filter (use 250 Hz filter for lead wire)
From Table 3.15, Channel Update Time vs. Filter Frequency, on page 3-27:
1. Calculate Module Update Time without a Lead Wire Compensation Cycle
= Ch 0 Update Time + Ch 1 Update Time = 53 ms + 15 ms = 68 ms
2. Calculate Module Update Time during a Lead Wire Compensation Cycle
Channel 0 Scan 1 (Module Scan 1)
Ch 0 Update Time + Ch 0 Lead Wire Compensation Time + Ch 1 Update Time
= 53 ms + 53 ms + 15 ms = 121 ms
The above module update time impact lasts for one more module scan, before the lead-wire
compensation cycle is complete for Channel 0:
Channel 0 Lead Wire Compensation Cycle Time
= (2 x 121 ms) = 242 ms
After that, a 2-scan lead wire cycle begins for Channel 1:
Channel 1 Scan 1 (Module Scan 3)
= Ch 0 Update Time + Ch 1 Update Time + Ch 1 Lead Wire Compensation Time
= 53 ms + 15 ms + 15 ms = 83 ms
The above module update time impact lasts for one more module scan, before the lead-wire
compensation cycle is complete for Channel 1:
Channel 1 Lead Wire Compensation Cycle Time
= (2 x 83 ms) = 166 ms
3. Calculate Total Time to Complete Lead Wire Compensation Cycle
= (Ch 0 Lead Wire Compensation Cycle Time) + (Ch 1 Lead Wire Compensation Cycle Time)
= (242 ms) + (166 ms)
= 408 ms = 0.408 seconds
After the above cycles are complete, the module returns to scans without lead wire compensation
for approximately 5 minutes. At that time, the lead wire compensation cycle repeats.
If both cyclic autocalibration (see page 3-28) and lead wire compensation are enabled, the two
functions run concurrent to one another.