BusWorks® 965EN Module User’s Manual Modbus TCP/IP mV/TC Input
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Acromag, Inc. Tel:248-295-0880 Fax:248-624-9234 Email:
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The 965EN-2004 model includes two CJC reference sensors. The 965EN-
2006 model includes three CJC reference sensors. These reference
sensors are calibrated separate from the inputs, but use the adjacent input
channels 0, 2, and 4, plus the ±1V and Type J TC ranges to accomplish
calibration. As such, be sure the Type J TC and ±1V ranges are already
calibrated at inputs 0, 2, and 4 prior to calibrating CJC. Then enable CJC
and calibrate Cold Junction Compensation as follows:
IMPORTANT:
For best results, allow the module to warm-up for 60 minutes
before calibrating cold junction compensation. Further, position the module
as in the final application during warmup. Ambient must be between 10°C
and 40°C.
CJC Calibration Via The Modbus TCP/IP Interface:
1. Write to the TC Break & CJC Configuration Register to turn CJC ON and
set the break detection as required by your application.
2. Write to the appropriate Input Range Register to select the TC Type J
range for the input channels adjacent to the CJC sensors (inputs 0, 2,
and 4)
—you will use these inputs to pass your CJC calibration signal.
3. Write 24106 (5E2AH) into the Calibration Access Register to remove
write protection from the calibration registers.
4. Connect a Type J TC reference at 0°C (0.000mV) to inputs 0, 2, and 4
and allow the input to settle about 10 seconds.
5. Write a 16-bit value to the Tref Calibration Register with a set bit in the
bit position that corresponds to the CJC channel to be calibrated (one
channel at a time). If you are calibrating CJC0, you would write 0x0001
to the Tref Calibration Register. If calibrating CJC1, write 0x0002. If
calibrating CJC2, write 0x0004. Note that the module will replace the
calibration coefficients immediately and no reset is needed. Further, no
response will be received and Modbus will timeout
—this is normal, as it
takes several seconds to process your calibration.
If calibration is successful, the input adjacent to the CJC will read 0.0°C
±0.1°C after about 10 seconds (this is input 0, 2, or 4 for CJC 0, 1, or 2).
If calibration is not successful, the input adjacent to the CJC will read
1000.0°C (this is input 0, 2, or 4 for CJC 0, 1, or 2) and you must try to
calibrate again. First, check that your input is a Type J TC, your module
is warmed-up, your signal is 0°C (0.000mV), and you are at the correct
input channel. Then retry calibration.
6. Repeat step 4-5 for the other CJC sensors.
7. When finished calibrating, write 0x0000 to the Calibration Access
Register (Holding register 21) to replace write protection to the
calibration registers and prevent miscalibration.
CJC Calibration
In order to calibrate CJC, the
module ambient must be within
10°C to 40°C. Do not attempt
to recalibrate CJC outside of,
or near these end points, as
this may negatively affect
module accuracy.
