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KB82443
SNAP-AIRTD Generic Input resistance (ohm) measurement needs additional calculations
Revision:  1.0
Published:  1/22/2013
Applies To
Firmware for:
SNAP-PAC-EB1
SNAP-PAC-EB2
SNAP-PAC-EB1-W
SNAP-PAC-EB2-W
SNAP-PAC-SB1
SNAP-PAC-SB2
SNAP-PAC-R1
SNAP-PAC-R2
SNAP-PAC-R1-W
SNAP-PAC-R2-W
SNAP-B3000-ENET
SNAP-UP1-ADS
SNAP-UP1-M64
Versions
Versions affected:  
Firmware R9.3c or earlier
Problem is improved in version:  
R9.3e for PAC units

SYMPTOMS

If a SNAP-AIRTD module is configured as a Generic Input (0x01) Point Type in order to read the RTD Resistance (ohms), the Scaled Units (ohms) and the Raw Counts values are inaccurate; they erroneously include half of the lead resistance. In order to obtain accurate values, it is necessary to read the Raw Counts value and perform some mathematical processing.

WORKAROUND

For firmware R9.3c or earlier.
Read the point's Raw Counts from the Opto Memory Map and perform the following calculations:

Compensated RTD Resistance Scaled Units (Ohms) = [ (Raw Counts / 65536) - (Raw Counts bit-wise AND 65535) ] * SNAP-SCALING

Uncompensated RTD Resistance Scaled Units (Ohms) = [ (Raw Counts / 65536) + (Raw Counts bit-wise AND 65535) ] * SNAP-SCALING

where SNAP-SCALING = module input range / 25000. See the specific module data sheet for the input range.

You can find the memory map address of the Raw Counts for each point by using PAC Manager

For firmware R9.3e
The Raw Counts field and Scaled Units (ohms) are uncompensated values (they include the full lead resistance). PAC Control 9.3 and prior does not have the ability to put the point into compensated mode (with the lead resistance removed). PAC Control 9.4 will provide this capability in a future release.

Read the point's Raw Counts from the Opto Memory Map and perform the following calculations:

Compensated RTD Resistance Scaled Units (Ohms) = [ (Raw Counts / 65536) - (2 * (Raw Counts bit-wise AND 65535)) ] * SNAP-SCALING

where SNAP-SCALING = module input range / 25000. See the specific module data sheet for the input range.

CASE STATUS

Opto 22 is investigating this issue. The full solution will include PAC firmware R9.3e along with PAC Control 9.4, which will be available in a future release.

Relevant Downloads
Questions? Contact Opto 22 Product Support.
Phone: 800-835-6786 or 951-695-3080
Email: support@opto22.com

DISCLAIMER

This Opto 22 Knowledge Base ('OptoKB') article is intended to provide general technical information on a particular subject or subjects and is not an exhaustive treatment of such subjects. Accordingly, the information in this OptoKB article is not intended to constitute application, design, software, or other professional engineering advice or services. Opto 22 may modify the OptoKB articles at any time. Before making any decision or taking any action which might affect your equipment, you should consult a qualified professional.

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