Rockwell Automation 1771-IR Series D RTD Input Module User Manual - Manual (Page 45)

6–7 Calibrating Your Module
Publication 1771Ć6.5.129 - March 2000
Offset Calibration
1. Attach the 1.00 ohm, 1% resistors as shown in Figure 6.1.
2. Examine word 3 (channel 1 data) in the read block transfer file.
Note the value. It should be around 1.00 (100 for 10 mohm
resolution; 33 for 30 mohm resolution).
3. Examine word 9 of the write block transfer data file. Bits 16–10
make up the offset correction byte. Bit 17 is the sign bit.
4. Subtract the data value that you noted in step 2 from 100. The
difference should be within +127 to –127. If it is not, the required
correction is beyond the range of software calibration. If the
difference is within range, input the difference (positive or
negative), in binary form, in bits 17–10 of word 9 in the write
block transfer file.
For example, if, at 1.00 ohm, word 3 of the read block transfer
data file shows 147, you would subtract 147 from 100, which
equals –47. You would then enter 10101111 (–47) in the upper
byte of word 9. The leading 1 (bit 17) is the polarity bit. It
indicates a negative correction factor. That is, you want to
subtract 47 counts from your input data. The lower byte remains
00 during offset calibration.
5. Repeat above steps for channels 2 through 6 respectively.
6. Apply the values by sending a BTW to the module.
Gain Calibration
1. Connect the 402.00, .01% resistors to the swing arm as shown in
Figure 6.2.
2. Place the module in platinum ohm mode. This provides 30 mohm
resolution display.
3. Examine word 3 of the read block transfer data file. It should be
around 13400 decimal. Your actual value will be a percentage of
13400.
For example, if the data in word 3 is 13408, then:
(13400–13408)/134000 = –0.000597.
Your actual data value differs from the theoretical value (at
402.0 ohms input resistance) by –0.000597, or –0.0597%.
You can compensate for this error by entering the percentage
difference in binary coded fraction form. Table 6.A lists the
value for bits 7–0.